Table of Contents
+ +Table of Contents
+ +Strings were briefly introduced previously in the introduction document. In this +section strings will be presented in greater detail. All the standard operations +that can be performed on sequences such as indexing, slicing, multiplication, length +minimum and maximum can be performed on string variables as well. One thing to +be noted is that strings are immutable, which means that string variables are +unchangeable. Hence, all item and slice assignments on strings are illegal. +Let us look at a few example.
+>>> name = 'PythonFreak' +>>> print name[3] +h +>>> print name[-1] +k +>>> print name[6:] +Freak +>>> name[6:0] = 'Maniac' +Traceback (most recent call last): + File "<stdin>", line 1, in <module> +TypeError: 'str' object does not support item assignment+
This is quite expected, since string objects are immutable as already mentioned. +The error message is clear in mentioning that 'str' object does not support item +assignment.
+String formatting can be performed using the string formatting operator represented +as the percent (%) sign. The string placed before the % sign is formatted with +the value placed to the right of it. Let us look at a simple example.
+>>> format = 'Hello %s, from PythonFreak' +>>> str1 = 'world!' +>>> print format % str1 +Hello world!, from PythonFreak+
The %s parts of the format string are called the coversion specifiers. The coversion +specifiers mark the places where the formatting has to be performed in a string. +In the example the %s is replaced by the value of str1. More than one value can +also be formatted at a time by specifying the values to be formatted using tuples +and dictionaries (explained in later sections). Let us look at an example.
+ >>> format = 'Hello %s, from %s'
+>>> values = ('world!', 'PythonFreak')
+>>> print format % values
+Hello world!, from PythonFreak
+In this example it can be observed that the format string contains two conversion +specifiers and they are formatted using the tuple of values as shown.
+The s in %s specifies that the value to be replaced is of type string. Values of +other types can be specified as well such as integers and floats. Integers are +specified as %d and floats as %f. The precision with which the integer or the +float values are to be represented can also be specified using a . (dot) +followed by the precision value.
+Similar to list methods, strings also have a rich set of methods to perform various +operations on strings. Some of the most important and popular ones are presented +in this section.
+The find method is used to search for a substring within a given string. It +returns the left most index of the first occurence of the substring. If the +substring is not found in the string then it returns -1. Let us look at a few +examples.
+ >>> longstring = 'Hello world!, from PythonFreak'
+>>> longstring.find('Python')
+19
+>>> longstring.find('Perl')
+-1
+The join method is used to join the elements of a sequence. The sequence +elements that are to be join ed should all be strings. Let us look at a few +examples.
+>>> seq = ['With', 'great', 'power', 'comes', 'great', 'responsibility'] +>>> sep = ' ' +>>> sep.join(seq) +'With great power comes great responsibility' +>>> sep = ',!' +>>> sep.join(seq) +'With,!great,!power,!comes,!great,!responsibility'+
Try this yourself
+>>> seq = [12,34,56,78] +>>> sep.join(seq)+
The lower method, as the name indicates, converts the entire text of a string +to lower case. It is specially useful in cases where the programmers deal with case +insensitive data. Let us look at a few examples.
+>>> sometext = 'Hello world!, from PythonFreak' +>>> sometext.lower() +'hello world!, from pythonfreak'+
The replace method replaces a substring with another substring within +a given string and returns the new string. Let us look at an example.
+ >>> sometext = 'Concise, precise and criticise is some of the words that end with ise'
+>>> sometext.replace('is', 'are')
+'Concaree, precaree and criticaree are some of the words that end with aree'
+Observe here that all the occurences of the substring is have been replaced, +even the is in concise, precise and criticise have been replaced.
+The split is one of the very important string methods. split is the opposite of the +join method. It is used to split a string based on the argument passed as the +delimiter. It returns a list of strings. By default when no argument is passed it +splits with space (' ') as the delimiter. Let us look at an example.
+ >>> grocerylist = 'butter, cucumber, beer(a grocery item??), wheatbread'
+>>> grocerylist.split(',')
+['butter', ' cucumber', ' beer(a grocery item??)', ' wheatbread']
+>>> grocerylist.split()
+['butter,', 'cucumber,', 'beer(a', 'grocery', 'item??),', 'wheatbread']
+Observe here that in the second case when the delimiter argument was not set +split was done with space as the delimiter.
+The strip method is used to remove or strip off any whitespaces that exist +to the left and right of a string, but not the whitespaces within a string. Let +us look at an example.
+>>> spacedtext = " Where's the text?? " +>>> spacedtext.strip() +"Where's the text??"+
Observe that the whitespaces between the words have not been removed.
+Note: Very important thing to note is that all the methods shown above do not + transform the source string. The source string still remains the same. + Remember that **strings are immutable**.+
Python is often referred to as a "Batteries included!" language, mainly because +of the Python Standard Library. The Python Standard Library provides an extensive +set of features some of which are available directly for use while some require to +import a few modules. The Standard Library provides various built-in functions +like:
+abs()
dict()
enumerate()
The built-in constants like True and False are provided by the Standard Library. +More information about the Python Standard Library is available
+Files are very important aspects when it comes to computing and programming. +Up until now the focus has been on small programs that interacted with users +through input() and raw_input(). Generally, for computational purposes +it becomes necessary to handle files, which are usually large in size as well. +This section focuses on basics of file handling.
+Files can be opened using the open() method. open() accepts 3 arguments +out of which 2 are optional. Let us look at the syntax of open():
+f = open( filename, mode, buffering)
+The filename is a compulsory argument while the mode and buffering are +optional. The filename should be a string and it should be the complete path +to the file to be opened (The path can be absolute or relative). Let us look at +an example.
+ >>> f = open ('basic_python/interim_assessment.rst')
+The mode argument specifies the mode in which the file has to be opened. +The following are the valid mode arguments:
+r - Read mode +w - Write mode +a - Append mode +b - Binary mode ++ - Read/Write mode
+The read mode opens the file as a read-only document. The write mode opens the +file in the Write only mode. In the write mode, if the file existed prior to the +opening, the previous contents of the file are erased. The append mode opens the +file in the write mode but the previous contents of the file are not erased and +the current data is appended onto the file. +The binary and the read/write modes are special in the sense that they are added +onto other modes. The read/write mode opens the file in the reading and writing +mode combined. The binary mode can be used to open a files that do not contain +text. Binary files such as images should be opened in the binary mode. Let us look +at a few examples.
+ >>> f = open ('basic_python/interim_assessment.rst', 'r')
+>>> f = open ('armstrong.py', 'r+')
+The third argument to the open() method is the buffering argument. This takes +a boolean value, True or 1 indicates that buffering has to be enabled on the file, +that is the file is loaded on to the main memory and the changes made to the file are +not immediately written to the disk. If the buffering argument is 0 or False the +changes are directly written on to the disk immediately.
+write(), evidently, is used to write data onto a file. It takes the data to +be written as the argument. The data can be a string, an integer, a float or any +other datatype. In order to be able to write data onto a file, the file has to +be opened in one of w, a or + modes.
+read() is used to read data from a file. It takes the number of bytes of data +to be read as the argument. If nothing is specified by default it reads the entire +contents from the current position to the end of file.
+Let us look at a few examples:
+ >>> f = open ('randomtextfile', 'w')
+>>> f.write('Hello all, this is PythonFreak. This is a random text file.')
+>>> f = open ('../randomtextfile', 'r')
+>>> f = open ('../randomtextfile', 'r')
+>>> f.read(5)
+'Hello'
+>>> f.read()
+' all, this is PythonFreak. This is a random text file.'
+>>> f.close()
+readline() is used to read a file line by line. readline() reads a line +of a file at a time. When an argument is passed to readline() it reads that +many bytes from the current line.
+One other method to read a file line by line is using the read() and the +for construct. Let us look at this block of code as an example.
+ >>> f = open('../randomtextfile', 'r')
+>>> for line in f:
+... print line
+...
+Hello all!
+
+This is PythonFreak on the second line.
+
+This is a random text file on line 3
+One must always close all the files that have been opened. Although, files opened +will be closed automatically when the program ends. When files opened in read mode +are not closed it might lead to uselessly locked sometimes. In case of files +opened in the write mode it is more important to close the files. This is because, +Python maybe using the file in the buffering mode and when the file is not closed +the buffer maybe lost completely and the changes made to the file are lost forever.
+A dictionary in general, are designed to be able to look up meanings of words. +Similarly, the Python dictionaries are also designed to look up for a specific +key and retrieve the corresponding value. Dictionaries are data structures that +provide key-value mappings. Dictionaries are similar to lists except that instead +of the values having integer indexes, dictionaries have keys or strings as indexes. +Let us look at an example of how to define dictionaries.
+ >>> dct = { 'Sachin': 'Tendulkar', 'Rahul': 'Dravid', 'Anil': 'Kumble'}
+The dictionary consists of pairs of strings, which are called keys and their +corresponding values separated by : and each of these key-value pairs are +comma(',') separated and the entire structure wrapped in a pair curly braces {}.
+Note: The data inside a dictionary is not ordered. The order in which you enter +the key-value pairs is not the order in which they are stored in the dictionary. +Python has an internal storage mechanism for that which is out of the purview +of this document.+
The dict() function is used to create dictionaries from other mappings or other +dictionaries. Let us look at an example.
+>>> diction = dict(mat = 133, avg = 52.53)+
String Formatting with Dictionaries:
+String formatting was discussed in the previous section and it was mentioned that +dictionaries can also be used for formatting more than one value. This section +focuses on the formatting of strings using dictionaries. String formatting using +dictionaries is more appealing than doing the same with tuples. Here the keyword +can be used as a place holder and the value corresponding to it is replaced in +the formatted string. Let us look at an example.
+ >>> player = { 'Name':'Rahul Dravid', 'Matches':133, 'Avg':52.53, '100s':26 }
+>>> strng = '%(Name)s has played %(Matches)d with an average of %(Avg).2f and has %(100s)d hundreds to his name.'
+>>> print strng % player
+Rahul Dravid has played 133 with an average of 52.53 and has 26 hundreds to his name.
+The clear() method removes all the existing key-value pairs from a dictionary. +It returns None or rather does not return anything. It is a method that changes +the object. It has to be noted here that dictionaries are not immutable. Let us +look at an example.
+ >>> dct
+{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
+>>> dct.clear()
+>>> dct
+{}
+The copy() returns a copy of a given dictionary. Let us look at an example.
+ >>> dct = {'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
+>>> dctcopy = dct.copy()
+>>> dctcopy
+{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
+get() returns the value for the key passed as the argument and if the +key does not exist in the dictionary, it returns None. Let us look at an +example.
+ >>> print dctcopy.get('Saurav')
+None
+>>> print dctcopy.get('Anil')
+Kumble
+This method returns True if the given key is in the dictionary, else it returns +False.
+ >>> dctcopy.has_key('Saurav')
+False
+>>> dctcopy.has_key('Sachin')
+True
+This method is used to retrieve the value of a given key and subsequently +remove the key-value pair from the dictionary. Let us look at an example.
+ >>> print dctcopy.pop('Sachin')
+Tendulkar
+>>> dctcopy
+{'Anil': 'Kumble', 'Rahul': 'Dravid'}
+This method randomly pops a key-value pair from a dictionary and returns it. +The key-value pair returned is removed from the dictionary. Let us look at an +example.
+ >>> print dctcopy.popitem()
+('Anil', 'Kumble')
+>>> dctcopy
+{'Rahul': 'Dravid'}
+
+Note that the item chosen is completely random since dictionaries are unordered
+as mentioned earlier.
+The update() method updates the contents of one dictionary with the contents +of another dictionary. For items with existing keys their values are updated, +and the rest of the items are added. Let us look at an example.
+ >>> dctcopy.update(dct)
+>>> dct
+{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
+>>> dctcopy
+{'Anil': 'Kumble', 'Sachin': 'Tendulkar', 'Rahul': 'Dravid'}
+