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| changeset 37 | 33abbab466a2 |
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<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Chapter. strings_dicts</title><link rel="stylesheet" href="/review/support/styles.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.74.3" /><link rel="shortcut icon" type="image/png" href="/review/support/figs/favicon.png" /><script type="text/javascript" src="/review/support/jquery-min.js"></script><script type="text/javascript" src="/review/support/form.js"></script><script type="text/javascript" src="/review/support/hsbook.js"></script></head> |
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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="#id2983086">Strings</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id2981673">1. String Formatting</a></span></dt> |
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<dt><span class="section"><a href="#id3034090">2. String Methods</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id3034105">2.1. <span class="strong"><strong>find</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034135">2.2. <span class="strong"><strong>join</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034175">2.3. <span class="strong"><strong>lower</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034206">2.4. <span class="strong"><strong>replace</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034262">2.5. <span class="strong"><strong>split</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034321">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="#id3034366">3. Introduction to the standard library</a></span></dt> |
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<dt><span class="section"><a href="#id3034442">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="#id3034472">4.1. Opening Files</a></span></dt> |
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<dt><span class="section"><a href="#id3034646">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="#id3034811">5. Dictionaries</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id3034868">5.1. <span class="strong"><strong>dict()</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034932">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="id2983086"></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="#id2981673">1. String Formatting</a></span></dt> |
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<dt><span class="section"><a href="#id3034090">2. String Methods</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id3034105">2.1. <span class="strong"><strong>find</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034135">2.2. <span class="strong"><strong>join</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034175">2.3. <span class="strong"><strong>lower</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034206">2.4. <span class="strong"><strong>replace</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034262">2.5. <span class="strong"><strong>split</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034321">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="#id3034366">3. Introduction to the standard library</a></span></dt> |
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<dt><span class="section"><a href="#id3034442">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="#id3034472">4.1. Opening Files</a></span></dt> |
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<dt><span class="section"><a href="#id3034646">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="#id3034811">5. Dictionaries</a></span></dt> |
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<dd><dl> |
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<dt><span class="section"><a href="#id3034868">5.1. <span class="strong"><strong>dict()</strong></span></a></span></dt> |
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<dt><span class="section"><a href="#id3034932">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">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"> |
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>>> name = 'PythonFreak' |
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>>> print name[3] |
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h |
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>>> print name[-1] |
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k |
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>>> print name[6:] |
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Freak |
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>>> name[6:0] = 'Maniac' |
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Traceback (most recent call last): |
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File "<stdin>", line 1, in <module> |
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TypeError: 'str' object does not support item assignment</pre> |
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<p id="ch4strings_dicts_2">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="id2981673"></a>1.String Formatting</h2></div></div></div> |
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<p id="ch4strings_dicts_3">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"> |
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>>> format = 'Hello %s, from PythonFreak' |
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>>> str1 = 'world!' |
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>>> print format % str1 |
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Hello world!, from PythonFreak</pre> |
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<p id="ch4strings_dicts_4">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"> |
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>>> format = 'Hello %s, from %s' |
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>>> values = ('world!', 'PythonFreak')
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>>> print format % values |
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Hello world!, from PythonFreak</pre> |
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<p id="ch4strings_dicts_5">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">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="id3034090"></a>2.String Methods</h2></div></div></div> |
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<p id="ch4strings_dicts_7">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="id3034105"></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">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"> |
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>>> longstring = 'Hello world!, from PythonFreak' |
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>>> longstring.find('Python')
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>>> 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="id3034135"></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">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"> |
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>>> seq = ['With', 'great', 'power', 'comes', 'great', 'responsibility'] |
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>>> sep = ' ' |
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>>> sep.join(seq) |
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'With great power comes great responsibility' |
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>>> sep = ',!' |
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>>> sep.join(seq) |
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'With,!great,!power,!comes,!great,!responsibility'</pre> |
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<p id="ch4strings_dicts_a"><span class="emphasis"><em>Try this yourself</em></span></p> |
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<pre class="programlisting"> |
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>>> seq = [12,34,56,78] |
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>>> 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="id3034175"></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">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"> |
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>>> sometext = 'Hello world!, from PythonFreak' |
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>>> 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="id3034206"></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">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"> |
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>>> sometext = 'Concise, precise and criticise is some of the words that end with ise' |
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>>> 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">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="id3034262"></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">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"> |
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>>> grocerylist = 'butter, cucumber, beer(a grocery item??), wheatbread' |
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>>> grocerylist.split(',')
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['butter', ' cucumber', ' beer(a grocery item??)', ' wheatbread'] |
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>>> grocerylist.split() |
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['butter,', 'cucumber,', 'beer(a', 'grocery', 'item??),', 'wheatbread']</pre> |
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<p id="ch4strings_dicts_f">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="id3034321"></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">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 |
| 28 | 206 |
us look at an example.</p> |
207 |
<pre class="programlisting"> |
|
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>>> spacedtext = " Where's the text?? " |
|
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>>> spacedtext.strip() |
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"Where's the text??"</pre> |
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<p id="ch4strings_dicts_11">Observe that the whitespaces between the words have not been removed.</p> |
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<pre class="programlisting"> |
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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="id3034366"></a>3.Introduction to the standard library</h2></div></div></div> |
|
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<p id="ch4strings_dicts_12">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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<div class="itemizedlist"><ul class="itemizedlist" type="*"> |
|
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<li class="listitem" style="list-style-type: *"><p id="ch4strings_dicts_13"><span class="strong"><strong>abs()</strong></span></p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch4strings_dicts_14"><span class="strong"><strong>dict()</strong></span></p></li> |
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<li class="listitem" style="list-style-type: *"><p id="ch4strings_dicts_15"><span class="strong"><strong>enumerate()</strong></span></p></li> |
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</ul></div> |
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<p id="ch4strings_dicts_16">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="id3034442"></a>4.I/O: Reading and Writing Files</h2></div></div></div> |
|
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<p id="ch4strings_dicts_17">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="id3034472"></a>4.1.Opening Files</h3></div></div></div> |
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<p id="ch4strings_dicts_18">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"><span class="emphasis"><em>f = open( filename, mode, buffering)</em></span></p> |
|
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<p id="ch4strings_dicts_1a">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"> |
|
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>>> f = open ('basic_python/interim_assessment.rst')</pre>
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<p id="ch4strings_dicts_1b">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"><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">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"> |
|
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>>> f = open ('basic_python/interim_assessment.rst', 'r')
|
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>>> f = open ('armstrong.py', 'r+')</pre>
|
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<p id="ch4strings_dicts_1e">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 |
280 |
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="id3034646"></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="id3034655"></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"><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> |
293 |
</div> |
|
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<div class="section" title="4.2.2.read()"> |
|
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<div class="titlepage"><div><div><h4 class="title"> |
|
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<a name="id3034694"></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"><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 |
| 28 | 300 |
contents from the current position to the end of file.</p> |
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<p id="ch4strings_dicts_21">Let us look at a few examples:</p> |
|
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<pre class="programlisting"> |
|
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>>> f = open ('randomtextfile', 'w')
|
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>>> f.write('Hello all, this is PythonFreak. This is a random text file.')
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>>> f = open ('../randomtextfile', 'r')
|
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>>> f = open ('../randomtextfile', 'r')
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>>> f.read(5) |
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'Hello' |
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>>> f.read() |
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' all, this is PythonFreak. This is a random text file.' |
| 28 | 311 |
>>> f.close()</pre> |
312 |
</div> |
|
313 |
<div class="section" title="4.2.3.readline()"> |
|
314 |
<div class="titlepage"><div><div><h4 class="title"> |
|
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<a name="id3034729"></a>4.2.3.<span class="strong"><strong>readline()</strong></span> |
|
316 |
</h4></div></div></div> |
|
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<p id="ch4strings_dicts_22"><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> |
|
320 |
<p id="ch4strings_dicts_23">One other method to read a file line by line is using the <span class="strong"><strong>read()</strong></span> and the |
|
321 |
<span class="strong"><strong>for</strong></span> construct. Let us look at this block of code as an example.</p> |
|
322 |
<pre class="programlisting"> |
|
323 |
>>> f = open('../randomtextfile', 'r')
|
|
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>>> for line in f: |
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... print line |
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... |
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Hello all! |
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|
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This is PythonFreak on the second line. |
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|
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This is a random text file on line 3</pre> |
332 |
</div> |
|
333 |
<div class="section" title="4.2.4.close()"> |
|
334 |
<div class="titlepage"><div><div><h4 class="title"> |
|
335 |
<a name="id3034788"></a>4.2.4.<span class="strong"><strong>close()</strong></span> |
|
336 |
</h4></div></div></div> |
|
337 |
<p id="ch4strings_dicts_24">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 |
| 28 | 342 |
the buffer maybe lost completely and the changes made to the file are lost forever.</p> |
343 |
</div> |
|
344 |
</div> |
|
345 |
</div> |
|
346 |
<div class="section" title="5.Dictionaries"> |
|
347 |
<div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
348 |
<a name="id3034811"></a>5.Dictionaries</h2></div></div></div> |
|
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<p id="ch4strings_dicts_25">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. |
| 28 | 354 |
Let us look at an example of how to define dictionaries.</p> |
355 |
<pre class="programlisting"> |
|
356 |
>>> dct = { 'Sachin': 'Tendulkar', 'Rahul': 'Dravid', 'Anil': 'Kumble'}</pre>
|
|
357 |
<p id="ch4strings_dicts_26">The dictionary consists of pairs of strings, which are called <span class="emphasis"><em>keys</em></span> and their |
|
358 |
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 |
|
359 |
comma(',') separated and the entire structure wrapped in a pair curly braces <span class="emphasis"><em>{}</em></span>.</p>
|
|
360 |
<pre class="programlisting"> |
|
361 |
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 |
| 28 | 364 |
of this document.</pre> |
365 |
<div class="section" title="5.1.dict()"> |
|
366 |
<div class="titlepage"><div><div><h3 class="title"> |
|
367 |
<a name="id3034868"></a>5.1.<span class="strong"><strong>dict()</strong></span> |
|
368 |
</h3></div></div></div> |
|
369 |
<p id="ch4strings_dicts_27">The <span class="strong"><strong>dict()</strong></span> function is used to create dictionaries from other mappings or other |
|
370 |
dictionaries. Let us look at an example.</p> |
|
371 |
<pre class="programlisting"> |
|
372 |
>>> diction = dict(mat = 133, avg = 52.53)</pre> |
|
373 |
<p id="ch4strings_dicts_28"><span class="strong"><strong>String Formatting with Dictionaries:</strong></span></p> |
|
374 |
<p id="ch4strings_dicts_29">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 |
| 28 | 377 |
dictionaries is more appealing than doing the same with tuples. Here the <span class="emphasis"><em>keyword</em></span> |
378 |
can be used as a place holder and the <span class="emphasis"><em>value</em></span> corresponding to it is replaced in |
|
379 |
the formatted string. Let us look at an example.</p> |
|
380 |
<pre class="programlisting"> |
|
381 |
>>> player = { 'Name':'Rahul Dravid', 'Matches':133, 'Avg':52.53, '100s':26 }
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>>> 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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>>> print strng % player |
| 28 | 384 |
Rahul Dravid has played 133 with an average of 52.53 and has 26 hundreds to his name.</pre> |
385 |
</div> |
|
386 |
<div class="section" title="5.2.Dictionary Methods"> |
|
387 |
<div class="titlepage"><div><div><h3 class="title"> |
|
388 |
<a name="id3034932"></a>5.2.Dictionary Methods</h3></div></div></div> |
|
389 |
<div class="section" title="5.2.1.clear()"> |
|
390 |
<div class="titlepage"><div><div><h4 class="title"> |
|
391 |
<a name="id3034940"></a>5.2.1.<span class="strong"><strong>clear()</strong></span> |
|
392 |
</h4></div></div></div> |
|
393 |
<p id="ch4strings_dicts_2a">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"> |
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>>> dct |
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{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
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>>> dct.clear() |
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>>> dct |
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{}</pre>
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</div> |
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<div class="section" title="5.2.2.copy()"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id3034976"></a>5.2.2.<span class="strong"><strong>copy()</strong></span> |
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</h4></div></div></div> |
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<p id="ch4strings_dicts_2b">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"> |
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>>> dct = {'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
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>>> dctcopy = dct.copy() |
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>>> dctcopy |
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{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}</pre>
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</div> |
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<div class="section" title="5.2.3.get()"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id3035006"></a>5.2.3.<span class="strong"><strong>get()</strong></span> |
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</h4></div></div></div> |
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<p id="ch4strings_dicts_2c"><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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<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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example.</p> |
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<pre class="programlisting"> |
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>>> print dctcopy.get('Saurav')
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None |
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>>> print dctcopy.get('Anil')
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Kumble</pre> |
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</div> |
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<div class="section" title="5.2.4.has_key()"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id3035044"></a>5.2.4.<span class="strong"><strong>has_key()</strong></span> |
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</h4></div></div></div> |
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<p id="ch4strings_dicts_2d">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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<span class="emphasis"><em>False</em></span>.</p> |
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<pre class="programlisting"> |
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>>> dctcopy.has_key('Saurav')
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False |
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>>> dctcopy.has_key('Sachin')
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True</pre> |
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</div> |
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<div class="section" title="5.2.5.pop()"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id3035074"></a>5.2.5.<span class="strong"><strong>pop()</strong></span> |
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</h4></div></div></div> |
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<p id="ch4strings_dicts_2e">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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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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<pre class="programlisting"> |
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>>> print dctcopy.pop('Sachin')
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Tendulkar |
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>>> dctcopy |
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{'Anil': 'Kumble', 'Rahul': 'Dravid'}</pre>
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</div> |
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<div class="section" title="5.2.6.popitem()"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id3035108"></a>5.2.6.<span class="strong"><strong>popitem()</strong></span> |
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</h4></div></div></div> |
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<p id="ch4strings_dicts_2f">This method randomly pops a <span class="emphasis"><em>key-value</em></span> pair from a dictionary and returns it. |
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The <span class="emphasis"><em>key-value</em></span> pair returned is removed from the dictionary. Let us look at an |
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example.</p> |
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<pre class="programlisting"> |
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>>> print dctcopy.popitem() |
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('Anil', 'Kumble')
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>>> dctcopy |
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{'Rahul': 'Dravid'}
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Note that the item chosen is completely random since dictionaries are unordered |
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as mentioned earlier.</pre> |
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</div> |
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<div class="section" title="5.2.7.update()"> |
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<div class="titlepage"><div><div><h4 class="title"> |
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<a name="id3035144"></a>5.2.7.<span class="strong"><strong>update()</strong></span> |
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</h4></div></div></div> |
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<p id="ch4strings_dicts_30">The <span class="strong"><strong>update()</strong></span> method updates the contents of one dictionary with the contents |
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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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and the rest of the items are added. Let us look at an example.</p> |
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<pre class="programlisting"> |
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>>> dctcopy.update(dct) |
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>>> dct |
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{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
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>>> dctcopy |
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{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}</pre>
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</div> |
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</div> |
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</div> |
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</div> |
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</div></body> |
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</html> |
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