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author amit@thunder
Mon, 25 Jan 2010 18:56:45 +0530
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<html>
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<title>Chapter 4. strings_dicts</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch4strings_dicts">
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<div class="titlepage"></div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl>
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<dt><span class="article"><a href="#id2983496">Strings</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="#id2982083">1. String Formatting</a></span></dt>
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<dt><span class="section"><a href="#id3034500">2. String Methods</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="#id3034515">2.1. <span class="strong"><strong>find</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034545">2.2. <span class="strong"><strong>join</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034585">2.3. <span class="strong"><strong>lower</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034615">2.4. <span class="strong"><strong>replace</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034672">2.5. <span class="strong"><strong>split</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034731">2.6. <span class="strong"><strong>strip</strong></span></a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="#id3034775">3. Introduction to the standard library</a></span></dt>
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<dt><span class="section"><a href="#id3034853">4. I/O: Reading and Writing Files</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="#id3034883">4.1. Opening Files</a></span></dt>
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<dt><span class="section"><a href="#id3035057">4.2. Reading and Writing files</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="#id3035222">5. Dictionaries</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="#id3035278">5.1. <span class="strong"><strong>dict()</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3035342">5.2. Dictionary Methods</a></span></dt>
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</dl></dd>
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</dl></dd>
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</dl>
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</div>
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<div class="article" title="Strings">
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<div class="titlepage">
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<div><div><h2 class="title">
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<a name="id2983496"></a>Strings</h2></div></div>
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<hr>
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</div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl>
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<dt><span class="section"><a href="#id2982083">1. String Formatting</a></span></dt>
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<dt><span class="section"><a href="#id3034500">2. String Methods</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="#id3034515">2.1. <span class="strong"><strong>find</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034545">2.2. <span class="strong"><strong>join</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034585">2.3. <span class="strong"><strong>lower</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034615">2.4. <span class="strong"><strong>replace</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034672">2.5. <span class="strong"><strong>split</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3034731">2.6. <span class="strong"><strong>strip</strong></span></a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="#id3034775">3. Introduction to the standard library</a></span></dt>
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<dt><span class="section"><a href="#id3034853">4. I/O: Reading and Writing Files</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="#id3034883">4.1. Opening Files</a></span></dt>
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<dt><span class="section"><a href="#id3035057">4.2. Reading and Writing files</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="#id3035222">5. Dictionaries</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="#id3035278">5.1. <span class="strong"><strong>dict()</strong></span></a></span></dt>
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<dt><span class="section"><a href="#id3035342">5.2. Dictionary Methods</a></span></dt>
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</dl></dd>
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</dl>
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</div>
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<p id="ch4strings_dicts_1"></a>Strings were briefly introduced previously in the introduction document. In this
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section strings will be presented in greater detail. All the standard operations
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that can be performed on sequences such as indexing, slicing, multiplication, length
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minimum and maximum can be performed on string variables as well. One thing to
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be noted is that strings are immutable, which means that string variables are
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unchangeable. Hence, all item and slice assignments on strings are illegal.
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Let us look at a few example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; name = 'PythonFreak'
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&gt;&gt;&gt; print name[3]
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h
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&gt;&gt;&gt; print name[-1]
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k
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&gt;&gt;&gt; print name[6:]
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Freak
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&gt;&gt;&gt; name[6:0] = 'Maniac'
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Traceback (most recent call last):
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  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
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TypeError: 'str' object does not support item assignment</pre>
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<p id="ch4strings_dicts_2"></a>This is quite expected, since string objects are immutable as already mentioned.
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The error message is clear in mentioning that 'str' object does not support item
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assignment.</p>
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<div class="section" title="1. String Formatting">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="id2982083"></a>1. String Formatting</h2></div></div></div>
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<p id="ch4strings_dicts_3"></a>String formatting can be performed using the string formatting operator represented
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as the percent (%) sign. The string placed before the % sign is formatted with
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the value placed to the right of it. Let us look at a simple example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; format = 'Hello %s, from PythonFreak'
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&gt;&gt;&gt; str1 = 'world!'
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&gt;&gt;&gt; print format % str1
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Hello world!, from PythonFreak</pre>
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<p id="ch4strings_dicts_4"></a>The %s parts of the format string are called the coversion specifiers. The coversion
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specifiers mark the places where the formatting has to be performed in a string.
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In the example the %s is replaced by the value of str1. More than one value can
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also be formatted at a time by specifying the values to be formatted using tuples
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and dictionaries (explained in later sections). Let us look at an example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; format = 'Hello %s, from %s'
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&gt;&gt;&gt; values = ('world!', 'PythonFreak')
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&gt;&gt;&gt; print format % values
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Hello world!, from PythonFreak</pre>
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<p id="ch4strings_dicts_5"></a>In this example it can be observed that the format string contains two conversion
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specifiers and they are formatted using the tuple of values as shown.</p>
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<p id="ch4strings_dicts_6"></a>The s in %s specifies that the value to be replaced is of type string. Values of
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other types can be specified as well such as integers and floats. Integers are
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specified as %d and floats as %f. The precision with which the integer or the
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float values are to be represented can also be specified using a <span class="strong"><strong>.</strong></span> (<span class="strong"><strong>dot</strong></span>)
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followed by the precision value.</p>
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</div>
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<div class="section" title="2. String Methods">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="id3034500"></a>2. String Methods</h2></div></div></div>
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<p id="ch4strings_dicts_7"></a>Similar to list methods, strings also have a rich set of methods to perform various
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operations on strings. Some of the most important and popular ones are presented
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in this section.</p>
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<div class="section" title="2.1. find">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3034515"></a>2.1. <span class="strong"><strong>find</strong></span>
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</h3></div></div></div>
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<p id="ch4strings_dicts_8"></a>The <span class="strong"><strong>find</strong></span> method is used to search for a substring within a given string. It
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returns the left most index of the first occurence of the substring. If the
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substring is not found in the string then it returns -1. Let us look at a few
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examples.</p>
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<pre class="programlisting"> &gt;&gt;&gt; longstring = 'Hello world!, from PythonFreak'
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&gt;&gt;&gt; longstring.find('Python')
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19
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&gt;&gt;&gt; longstring.find('Perl')
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-1</pre>
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</div>
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<div class="section" title="2.2. join">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3034545"></a>2.2. <span class="strong"><strong>join</strong></span>
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</h3></div></div></div>
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<p id="ch4strings_dicts_9"></a>The <span class="strong"><strong>join</strong></span> method is used to join the elements of a sequence. The sequence
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elements that are to be join ed should all be strings. Let us look at a few
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examples.</p>
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<pre class="programlisting"> &gt;&gt;&gt; seq = ['With', 'great', 'power', 'comes', 'great', 'responsibility']
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&gt;&gt;&gt; sep = ' '
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&gt;&gt;&gt; sep.join(seq)
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'With great power comes great responsibility'
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&gt;&gt;&gt; sep = ',!'
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&gt;&gt;&gt; sep.join(seq)
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'With,!great,!power,!comes,!great,!responsibility'</pre>
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<p id="ch4strings_dicts_a"></a><span class="emphasis"><em>Try this yourself</em></span></p>
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<pre class="programlisting"> &gt;&gt;&gt; seq = [12,34,56,78]
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&gt;&gt;&gt; sep.join(seq)</pre>
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</div>
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<div class="section" title="2.3. lower">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3034585"></a>2.3. <span class="strong"><strong>lower</strong></span>
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</h3></div></div></div>
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<p id="ch4strings_dicts_b"></a>The <span class="strong"><strong>lower</strong></span> method, as the name indicates, converts the entire text of a string
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to lower case. It is specially useful in cases where the programmers deal with case
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insensitive data. Let us look at a few examples.</p>
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<pre class="programlisting"> &gt;&gt;&gt; sometext = 'Hello world!, from PythonFreak'
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&gt;&gt;&gt; sometext.lower()
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'hello world!, from pythonfreak'</pre>
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</div>
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<div class="section" title="2.4. replace">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3034615"></a>2.4. <span class="strong"><strong>replace</strong></span>
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</h3></div></div></div>
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<p id="ch4strings_dicts_c"></a>The <span class="strong"><strong>replace</strong></span> method replaces a substring with another substring within
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a given string and returns the new string. Let us look at an example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; sometext = 'Concise, precise and criticise is some of the words that end with ise'
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&gt;&gt;&gt; sometext.replace('is', 'are')
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'Concaree, precaree and criticaree are some of the words that end with aree'</pre>
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<p id="ch4strings_dicts_d"></a>Observe here that all the occurences of the substring <span class="emphasis"><em>is</em></span> have been replaced,
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even the <span class="emphasis"><em>is</em></span> in <span class="emphasis"><em>concise</em></span>, <span class="emphasis"><em>precise</em></span> and <span class="emphasis"><em>criticise</em></span> have been replaced.</p>
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</div>
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<div class="section" title="2.5. split">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3034672"></a>2.5. <span class="strong"><strong>split</strong></span>
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</h3></div></div></div>
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<p id="ch4strings_dicts_e"></a>The <span class="strong"><strong>split</strong></span> is one of the very important string methods. split is the opposite of the
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<span class="strong"><strong>join</strong></span> method. It is used to split a string based on the argument passed as the
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delimiter. It returns a list of strings. By default when no argument is passed it
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splits with <span class="emphasis"><em>space</em></span> (' ') as the delimiter. Let us look at an example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; grocerylist = 'butter, cucumber, beer(a grocery item??), wheatbread'
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&gt;&gt;&gt; grocerylist.split(',')
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['butter', ' cucumber', ' beer(a grocery item??)', ' wheatbread']
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&gt;&gt;&gt; grocerylist.split()
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['butter,', 'cucumber,', 'beer(a', 'grocery', 'item??),', 'wheatbread']</pre>
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<p id="ch4strings_dicts_f"></a>Observe here that in the second case when the delimiter argument was not set
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<span class="strong"><strong>split</strong></span> was done with <span class="emphasis"><em>space</em></span> as the delimiter.</p>
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</div>
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<div class="section" title="2.6. strip">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3034731"></a>2.6. <span class="strong"><strong>strip</strong></span>
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</h3></div></div></div>
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<p id="ch4strings_dicts_10"></a>The <span class="strong"><strong>strip</strong></span> method is used to remove or <span class="strong"><strong>strip</strong></span> off any whitespaces that exist
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to the left and right of a string, but not the whitespaces within a string. Let
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us look at an example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; spacedtext = "               Where's the text??                 "
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&gt;&gt;&gt; spacedtext.strip()
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"Where's the text??"</pre>
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<p id="ch4strings_dicts_11"></a>Observe that the whitespaces between the words have not been removed.</p>
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<pre class="programlisting"> Note: Very important thing to note is that all the methods shown above do not
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      transform the source string. The source string still remains the same.
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      Remember that **strings are immutable**.</pre>
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</div>
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</div>
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<div class="section" title="3. Introduction to the standard library">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="id3034775"></a>3. Introduction to the standard library</h2></div></div></div>
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<p id="ch4strings_dicts_12"></a>Python is often referred to as a "Batteries included!" language, mainly because
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of the Python Standard Library. The Python Standard Library provides an extensive
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set of features some of which are available directly for use while some require to
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import a few <span class="strong"><strong>modules</strong></span>. The Standard Library provides various built-in functions
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like:</p>
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<span style="color: black"><span style="color: black"><span style="color: black"><p id="ch4strings_dicts_13"></a><span class="strong"><strong>abs()</strong></span></p></span><span style="color: black"><p id="ch4strings_dicts_14"></a><span class="strong"><strong>dict()</strong></span></p></span><span style="color: black"><p id="ch4strings_dicts_15"></a><span class="strong"><strong>enumerate()</strong></span></p></span></span></span><p id="ch4strings_dicts_16"></a>The built-in constants like <span class="strong"><strong>True</strong></span> and <span class="strong"><strong>False</strong></span> are provided by the Standard Library.
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More information about the Python Standard Library is available </p>
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<div class="reference">
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<div class="titlepage"><hr></div>http://docs.python.org/library/</div>
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</div>
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<div class="section" title="4. I/O: Reading and Writing Files">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="id3034853"></a>4. I/O: Reading and Writing Files</h2></div></div></div>
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<p id="ch4strings_dicts_17"></a>Files are very important aspects when it comes to computing and programming.
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Up until now the focus has been on small programs that interacted with users
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through <span class="strong"><strong>input()</strong></span> and <span class="strong"><strong>raw_input()</strong></span>. Generally, for computational purposes
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it becomes necessary to handle files, which are usually large in size as well.
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This section focuses on basics of file handling.</p>
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<div class="section" title="4.1. Opening Files">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3034883"></a>4.1. Opening Files</h3></div></div></div>
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<p id="ch4strings_dicts_18"></a>Files can be opened using the <span class="strong"><strong>open()</strong></span> method. <span class="strong"><strong>open()</strong></span> accepts 3 arguments
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out of which 2 are optional. Let us look at the syntax of <span class="strong"><strong>open()</strong></span>:</p>
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<p id="ch4strings_dicts_19"></a><span class="emphasis"><em>f = open( filename, mode, buffering)</em></span></p>
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<p id="ch4strings_dicts_1a"></a>The <span class="emphasis"><em>filename</em></span> is a compulsory argument while the <span class="emphasis"><em>mode</em></span> and <span class="emphasis"><em>buffering</em></span> are
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optional. The <span class="emphasis"><em>filename</em></span> should be a string and it should be the complete path
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to the file to be opened (The path can be absolute or relative). Let us look at
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an example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; f = open ('basic_python/interim_assessment.rst')</pre>
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<p id="ch4strings_dicts_1b"></a>The <span class="emphasis"><em>mode</em></span> argument specifies the mode in which the file has to be opened.
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The following are the valid mode arguments:</p>
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<p id="ch4strings_dicts_1c"></a><span class="strong"><strong>r</strong></span> - Read mode
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<span class="strong"><strong>w</strong></span> - Write mode
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<span class="strong"><strong>a</strong></span> - Append mode
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<span class="strong"><strong>b</strong></span> - Binary mode
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<span class="strong"><strong>+</strong></span> - Read/Write mode</p>
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<p id="ch4strings_dicts_1d"></a>The read mode opens the file as a read-only document. The write mode opens the
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file in the Write only mode. In the write mode, if the file existed prior to the
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opening, the previous contents of the file are erased. The append mode opens the
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file in the write mode but the previous contents of the file are not erased and
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the current data is appended onto the file.
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The binary and the read/write modes are special in the sense that they are added
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onto other modes. The read/write mode opens the file in the reading and writing
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mode combined. The binary mode can be used to open a files that do not contain
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text. Binary files such as images should be opened in the binary mode. Let us look
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at a few examples.</p>
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<pre class="programlisting"> &gt;&gt;&gt; f = open ('basic_python/interim_assessment.rst', 'r')
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&gt;&gt;&gt; f = open ('armstrong.py', 'r+')</pre>
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<p id="ch4strings_dicts_1e"></a>The third argument to the <span class="strong"><strong>open()</strong></span> method is the <span class="emphasis"><em>buffering</em></span> argument. This takes
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a boolean value, <span class="emphasis"><em>True</em></span> or <span class="emphasis"><em>1</em></span> indicates that buffering has to be enabled on the file,
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that is the file is loaded on to the main memory and the changes made to the file are
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not immediately written to the disk. If the <span class="emphasis"><em>buffering</em></span> argument is <span class="emphasis"><em>0</em></span> or <span class="emphasis"><em>False</em></span> the
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changes are directly written on to the disk immediately.</p>
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</div>
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<div class="section" title="4.2. Reading and Writing files">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id3035057"></a>4.2. Reading and Writing files</h3></div></div></div>
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<div class="section" title="4.2.1. write()">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="id3035066"></a>4.2.1. <span class="strong"><strong>write()</strong></span>
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</h4></div></div></div>
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<p id="ch4strings_dicts_1f"></a><span class="strong"><strong>write()</strong></span>, evidently, is used to write data onto a file. It takes the data to
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be written as the argument. The data can be a string, an integer, a float or any
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other datatype. In order to be able to write data onto a file, the file has to
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be opened in one of <span class="strong"><strong>w</strong></span>, <span class="strong"><strong>a</strong></span> or <span class="strong"><strong>+</strong></span> modes.</p>
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</div>
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<div class="section" title="4.2.2. read()">
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<a name="id3035105"></a>4.2.2. <span class="strong"><strong>read()</strong></span>
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</h4></div></div></div>
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<p id="ch4strings_dicts_20"></a><span class="strong"><strong>read()</strong></span> is used to read data from a file. It takes the number of bytes of data
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to be read as the argument. If nothing is specified by default it reads the entire
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contents from the current position to the end of file.</p>
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<p id="ch4strings_dicts_21"></a>Let us look at a few examples:</p>
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<pre class="programlisting"> &gt;&gt;&gt; f = open ('randomtextfile', 'w')
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&gt;&gt;&gt; f.write('Hello all, this is PythonFreak. This is a random text file.')
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&gt;&gt;&gt; f = open ('../randomtextfile', 'r')
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&gt;&gt;&gt; f = open ('../randomtextfile', 'r')
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&gt;&gt;&gt; f.read(5)
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'Hello'
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&gt;&gt;&gt; f.read()
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' all, this is PythonFreak. This is a random text file.'
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&gt;&gt;&gt; f.close()</pre>
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</div>
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<div class="section" title="4.2.3. readline()">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="id3035140"></a>4.2.3. <span class="strong"><strong>readline()</strong></span>
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</h4></div></div></div>
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<p id="ch4strings_dicts_22"></a><span class="strong"><strong>readline()</strong></span> is used to read a file line by line. <span class="strong"><strong>readline()</strong></span> reads a line
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of a file at a time. When an argument is passed to <span class="strong"><strong>readline()</strong></span> it reads that
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many bytes from the current line.</p>
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<p id="ch4strings_dicts_23"></a>One other method to read a file line by line is using the <span class="strong"><strong>read()</strong></span> and the
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<span class="strong"><strong>for</strong></span> construct. Let us look at this block of code as an example.</p>
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<pre class="programlisting"> &gt;&gt;&gt; f = open('../randomtextfile', 'r')
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&gt;&gt;&gt; for line in f:
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...     print line
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...
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Hello all!
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This is PythonFreak on the second line.













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This is a random text file on line 3</pre>













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</div>













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<div class="section" title="4.2.4. close()">













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<div class="titlepage"><div><div><h4 class="title">













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<a name="id3035199"></a>4.2.4. <span class="strong"><strong>close()</strong></span>













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</h4></div></div></div>













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<p id="ch4strings_dicts_24"></a>One must always close all the files that have been opened. Although, files opened













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will be closed automatically when the program ends. When files opened in read mode













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are not closed it might lead to uselessly locked sometimes. In case of files













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opened in the write mode it is more important to close the files. This is because,













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Python maybe using the file in the buffering mode and when the file is not closed













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the buffer maybe lost completely and the changes made to the file are lost forever.</p>













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</div>













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</div>













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</div>













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<div class="section" title="5. Dictionaries">













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<div class="titlepage"><div><div><h2 class="title" style="clear: both">













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<a name="id3035222"></a>5. Dictionaries</h2></div></div></div>













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<p id="ch4strings_dicts_25"></a>A dictionary in general, are designed to be able to look up meanings of words.













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Similarly, the Python dictionaries are also designed to look up for a specific













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key and retrieve the corresponding value. Dictionaries are data structures that













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provide key-value mappings. Dictionaries are similar to lists except that instead













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of the values having integer indexes, dictionaries have keys or strings as indexes.













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Let us look at an example of how to define dictionaries.</p>













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<pre class="programlisting"> &gt;&gt;&gt; dct = { 'Sachin': 'Tendulkar', 'Rahul': 'Dravid', 'Anil': 'Kumble'}</pre>













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<p id="ch4strings_dicts_26"></a>The dictionary consists of pairs of strings, which are called <span class="emphasis"><em>keys</em></span> and their













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corresponding <span class="emphasis"><em>values</em></span> separated by <span class="emphasis"><em>:</em></span> and each of these <span class="emphasis"><em>key-value</em></span> pairs are













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comma(',') separated and the entire structure wrapped in a pair curly braces <span class="emphasis"><em>{}</em></span>.</p>













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<pre class="programlisting"> Note: The data inside a dictionary is not ordered. The order in which you enter













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the key-value pairs is not the order in which they are stored in the dictionary.













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Python has an internal storage mechanism for that which is out of the purview













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of this document.</pre>













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<div class="section" title="5.1. dict()">













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<div class="titlepage"><div><div><h3 class="title">













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<a name="id3035278"></a>5.1. <span class="strong"><strong>dict()</strong></span>













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</h3></div></div></div>













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<p id="ch4strings_dicts_27"></a>The <span class="strong"><strong>dict()</strong></span> function is used to create dictionaries from other mappings or other













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dictionaries. Let us look at an example.</p>













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<pre class="programlisting"> &gt;&gt;&gt; diction = dict(mat = 133, avg = 52.53)</pre>













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<p id="ch4strings_dicts_28"></a><span class="strong"><strong>String Formatting with Dictionaries:</strong></span></p>













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<p id="ch4strings_dicts_29"></a>String formatting was discussed in the previous section and it was mentioned that













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dictionaries can also be used for formatting more than one value. This section













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focuses on the formatting of strings using dictionaries. String formatting using













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dictionaries is more appealing than doing the same with tuples. Here the <span class="emphasis"><em>keyword</em></span>













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can be used as a place holder and the <span class="emphasis"><em>value</em></span> corresponding to it is replaced in













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the formatted string. Let us look at an example.</p>













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<pre class="programlisting"> &gt;&gt;&gt; player = { 'Name':'Rahul Dravid', 'Matches':133, 'Avg':52.53, '100s':26 }













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&gt;&gt;&gt; strng = '%(Name)s has played %(Matches)d with an average of %(Avg).2f and has %(100s)d hundreds to his name.'













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&gt;&gt;&gt; print strng % player













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Rahul Dravid has played 133 with an average of 52.53 and has 26 hundreds to his name.</pre>













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</div>













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<div class="section" title="5.2. Dictionary Methods">













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<div class="titlepage"><div><div><h3 class="title">













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<a name="id3035342"></a>5.2. Dictionary Methods</h3></div></div></div>













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<div class="section" title="5.2.1. clear()">













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<div class="titlepage"><div><div><h4 class="title">













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<a name="id3035351"></a>5.2.1. <span class="strong"><strong>clear()</strong></span>













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</h4></div></div></div>













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<p id="ch4strings_dicts_2a"></a>The <span class="strong"><strong>clear()</strong></span> method removes all the existing <span class="emphasis"><em>key-value</em></span> pairs from a dictionary.













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It returns <span class="emphasis"><em>None</em></span> or rather does not return anything. It is a method that changes













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the object. It has to be noted here that dictionaries are not immutable. Let us













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look at an example.</p>













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<pre class="programlisting"> &gt;&gt;&gt; dct













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{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}













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&gt;&gt;&gt; dct.clear()













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   386


&gt;&gt;&gt; dct













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   387


{}</pre>













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   388


</div>













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   389


<div class="section" title="5.2.2. copy()">













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   390


<div class="titlepage"><div><div><h4 class="title">













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   391


<a name="id3035386"></a>5.2.2. <span class="strong"><strong>copy()</strong></span>













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   392


</h4></div></div></div>













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<p id="ch4strings_dicts_2b"></a>The <span class="strong"><strong>copy()</strong></span> returns a copy of a given dictionary. Let us look at an example.</p>













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<pre class="programlisting"> &gt;&gt;&gt; dct = {'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}













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&gt;&gt;&gt; dctcopy = dct.copy()













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   396


&gt;&gt;&gt; dctcopy













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   397


{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}</pre>













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   398


</div>













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   399


<div class="section" title="5.2.3. get()">













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   400


<div class="titlepage"><div><div><h4 class="title">













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   401


<a name="id3035417"></a>5.2.3. <span class="strong"><strong>get()</strong></span>













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   402


</h4></div></div></div>













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   403


<p id="ch4strings_dicts_2c"></a><span class="strong"><strong>get()</strong></span> returns the <span class="emphasis"><em>value</em></span> for the <span class="emphasis"><em>key</em></span> passed as the argument and if the













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   404


<span class="emphasis"><em>key</em></span> does not exist in the dictionary, it returns <span class="emphasis"><em>None</em></span>. Let us look at an













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   405


example.</p>













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   406


<pre class="programlisting"> &gt;&gt;&gt; print dctcopy.get('Saurav')













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   407


None













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   408


&gt;&gt;&gt; print dctcopy.get('Anil')













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   409


Kumble</pre>













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   410


</div>













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   411


<div class="section" title="5.2.4. has_key()">













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   412


<div class="titlepage"><div><div><h4 class="title">













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   413


<a name="id3035455"></a>5.2.4. <span class="strong"><strong>has_key()</strong></span>













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   414


</h4></div></div></div>













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   415


<p id="ch4strings_dicts_2d"></a>This method returns <span class="emphasis"><em>True</em></span> if the given <span class="emphasis"><em>key</em></span> is in the dictionary, else it returns













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   416


<span class="emphasis"><em>False</em></span>.</p>













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   417


<pre class="programlisting"> &gt;&gt;&gt; dctcopy.has_key('Saurav')













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   418


False













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   419


&gt;&gt;&gt; dctcopy.has_key('Sachin')













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   420


True</pre>













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   421


</div>













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   422


<div class="section" title="5.2.5. pop()">













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   423


<div class="titlepage"><div><div><h4 class="title">













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   424


<a name="id3035485"></a>5.2.5. <span class="strong"><strong>pop()</strong></span>













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   425


</h4></div></div></div>













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   426


<p id="ch4strings_dicts_2e"></a>This method is used to retrieve the <span class="emphasis"><em>value</em></span> of a given <span class="emphasis"><em>key</em></span> and subsequently













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   427


remove the <span class="emphasis"><em>key-value</em></span> pair from the dictionary. Let us look at an example.</p>













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   428


<pre class="programlisting"> &gt;&gt;&gt; print dctcopy.pop('Sachin')













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   429


Tendulkar













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   430


&gt;&gt;&gt; dctcopy













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   431


{'Anil': 'Kumble', 'Rahul': 'Dravid'}</pre>













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   432


</div>













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   433


<div class="section" title="5.2.6. popitem()">













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   434


<div class="titlepage"><div><div><h4 class="title">













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   435


<a name="id3035519"></a>5.2.6. <span class="strong"><strong>popitem()</strong></span>













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   436


</h4></div></div></div>













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   437


<p id="ch4strings_dicts_2f"></a>This method randomly pops a <span class="emphasis"><em>key-value</em></span> pair from a dictionary and returns it.













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   438


The <span class="emphasis"><em>key-value</em></span> pair returned is removed from the dictionary. Let us look at an













8083d21c0020
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diff
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   439


example.</p>













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   440


<pre class="programlisting"> &gt;&gt;&gt; print dctcopy.popitem()













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   441


('Anil', 'Kumble')













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   442


&gt;&gt;&gt; dctcopy













8083d21c0020
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   443


{'Rahul': 'Dravid'}













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   444














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   445


Note that the item chosen is completely random since dictionaries are unordered













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   446


as mentioned earlier.</pre>













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   447


</div>













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   448


<div class="section" title="5.2.7. update()">













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   449


<div class="titlepage"><div><div><h4 class="title">













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   450


<a name="id3035555"></a>5.2.7. <span class="strong"><strong>update()</strong></span>













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changeset




   451


</h4></div></div></div>













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   452


<p id="ch4strings_dicts_30"></a>The <span class="strong"><strong>update()</strong></span> method updates the contents of one dictionary with the contents













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   453


of another dictionary. For items with existing <span class="emphasis"><em>keys</em></span> their <span class="emphasis"><em>values</em></span> are updated,













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   454


and the rest of the items are added. Let us look at an example.</p>













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   455


<pre class="programlisting"> &gt;&gt;&gt; dctcopy.update(dct)













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changeset




   456


&gt;&gt;&gt; dct













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changeset




   457


{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}













8083d21c0020
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diff
changeset




   458


&gt;&gt;&gt; dctcopy













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changeset




   459


{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}</pre>













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diff
changeset




   460


</div>













8083d21c0020
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diff
changeset




   461


</div>













8083d21c0020
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diff
changeset




   462


</div>













8083d21c0020
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diff
changeset




   463


</div>













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   464


</div></body>













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   465


</html>















SEES-hacks