A Python 2.7 programming tutorial

Given some string value s, to produce a new string containing s centered in a string of length n, use this method call:

s.center(n)

This method takes one argument n, the size of the result. Examples:

>>> k = "Ni"
>>> k.center(5)
'  Ni '
>>> "<*>".center(12)
'    <*>     '

Note that in the first example we are asking Python to center the string "Ni" in a field of length 5. Clearly we can't center a 2-character string in 5 characters, so Python arbitrarily adds the leftover space character before the old value.

Another useful string method left-justifies a value in a field of a given length. The general form:

s.ljust(n)

For any string expression s, this method returns a new string containing the characters from s with enough spaces added after it to make a new string of length n.

>>> "Ni".ljust(4)
'Ni  '
>>> "Too long to fit".ljust(4)
'Too long to fit'

Note that the .ljust() method will never return a shorter string. If the length isn't enough, it just returns the original value.

There is a similar method that right-justifies a string value:

s.rjust(n)

This method returns a string with enough spaces added before the value to make a string of length n. As with the .ljust() method, it will never return a string shorter than the original.

>>> "Ni".rjust(4)
'  Ni'
>>> m = "floccinaucinihilipilification"
>>> m.rjust(4)
'floccinaucinihilipilification'

Sometimes you want to remove whitespace (spaces, tabs, and newlines) from a string. For a string s, use these methods to remove leading and trailing whitespace:

>>> spaceCase = ' \n \t Moon   \t\t '
>>> spaceCase
' \n \t Moon   \t\t '
>>> spaceCase.strip()
'Moon'
>>> spaceCase.lstrip()
'Moon   \t\t '
>>> spaceCase.rstrip()
' \n \t Moon'

The method s.count(t) searches string s to see how many times string t occurs in it.

>>> chiq = "banana"
>>> chiq
'banana'
>>> chiq.count("a")
3
>>> chiq.count("b")
1
>>> chiq.count("x")
0
>>> chiq.count("an")
2
>>> chiq.count("ana")
1

Note that this method does not count overlapping occurrences. Although the string "ana" occurs twice in string "banana", the occurrences overlap, so "banana".count("ana") returns only 1.

Use this method to search for a string t within a string s:

s.find(t)

If t matches any part of s, the method returns the position where the first match begins; otherwise, it returns -1.

>>> chiq
'banana'
>>> chiq.find ( "b" )
0
>>> chiq.find ( "a" )
1
>>> chiq.find ( "x" )
-1
>>> chiq.find ( "nan" )
2

If you need to find the last occurrence of a substring, use the similar method s.rfind(t), which returns the position where the last match starts, or -1 if there is no match.

>>> chiq.rfind("a")
5
>>> chiq[5]
'a'
>>> chiq.rfind("n")
4
>>> chiq.rfind("b")
0
>>> chiq.rfind("Waldo")
-1

You can check to see if a string s starts with a string t using a method call like this:

s.startswith(t)

This method returns True if s starts with a string that matches t; otherwise it returns False.

>>> chiq
'banana'
>>> chiq.startswith("b")
True
>>> chiq.startswith("ban")
True
>>> chiq.startswith ( 'Waldo' )
False

There is a similar method s.endswith(t) that tests whether string s ends with t:

>>> chiq.endswith("Waldo")
False
>>> chiq.endswith("a")
True
>>> chiq.endswith("nana")
True

The special values True and False are discussed later in Section 6.1, “Conditions and the bool type”.

The methods s.lower() and s.upper() are used to convert uppercase characters to lowercase, and vice versa, respectively.

>>> poet = 'E. E. Cummings'
>>> poet.upper()
'E. E. CUMMINGS'
>>> poet.lower()
'e. e. cummings'

Use the string methods in this section to test whether a string contains certain kinds of characters. Each of these methods is a predicate, that is, it asks a question and returns a value of True or False.

>>> mixed = 'abcDEFghi'
>>> mixed.isalpha()
True
>>> mixed.isupper()
False
>>> mixed.islower()
False
>>> "ABCDGOLDFISH".isupper()
True
>>> "lmno goldfish".islower()
True
>>> "abc $%&*(".islower()
True
>>> "abC $%&*(".islower()
False
>>> paradise = "87801"
>>> paradise.isalpha()
False
>>> paradise.isdigit()
True
>>> "abc123".isdigit()
False

The .split() method is used to break a string up into pieces wherever a certain string called the delimiter is found; it returns a list of strings containing the text between the delimiters. For example, suppose you have a string that contains a series of numbers separated by whitespace. A call to the .split() method on that string, with no arguments, returns a list of the parts of the string that are surrounded by whitespace.

>>> line = "  1.4   8.6  -23.49   "
>>> line.split()
['1.4', '8.6', '-23.49']

You can also specify a delimiter as the argument of the .split() method. Examples:

>>> s = 'farcical/aquatic/ceremony'
>>> s.split('/')
['farcical', 'aquatic', 'ceremony']
>>> "//a/b/".split('/')
['', '', 'a', 'b', '']
>>> "Stilton; Wensleydale; Cheddar;Edam".split("; ")
['Stilton', 'Wensleydale', 'Cheddar;Edam']

You may also provide a second argument that limits the number of pieces to be split from the string.

>>> t = 'a/b/c/d/e'
>>> t.split('/')
['a', 'b', 'c', 'd', 'e']
>>> t.split('/', 1)
['a', 'b/c/d/e']
>>> t.split('/', 3)
['a', 'b', 'c', 'd/e']