Type-Specific Methods

Every string operation we’ve studied so far is really a sequence operation—that is, these operations will work on other sequences in Python as well, including lists and tuples. In addition to generic sequence operations, though, strings also have operations all their own, available as methods—functions attached to the object, which are triggered with a call expression.

For example, the string find method is the basic substring search operation (it returns the offset of the passed-in substring, or −1 if it is not present), and the string replace method performs global searches and replacements:

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>>> S.find('pa')            # Find the offset of a substring
1
>>> S
'Spam'
>>> S.replace('pa', 'XYZ')  # Replace occurrences of a substring with another
'SXYZm'
>>> S
'Spam'

Again, despite the names of these string methods, we are not changing the original strings here, but creating new strings as the results—because strings are immutable, we have to do it this way. String methods are the first line of text-processing tools in Python. Other methods split a string into substrings on a delimiter (handy as a simple form of parsing), perform case conversions, test the content of the string (digits, letters, and so on), and strip whitespace characters off the ends of the string:

>>> line = 'aaa,bbb,ccccc,dd'
>>> line.split(',')          # Split on a delimiter into a list of substrings
['aaa', 'bbb', 'ccccc', 'dd']
>>> S = 'spam'
>>> S.upper()                # Upper- and lowercase conversions
'SPAM'

>>> S.isalpha()              # Content tests: isalpha, isdigit, etc.
True

>>> line = 'aaa,bbb,ccccc,dd\n'
>>> line = line.rstrip()     # Remove whitespace characters on the right side
>>> line
'aaa,bbb,ccccc,dd'

Strings also support an advanced substitution operation known as formatting, available as both an expression (the original) and a string method call (new in 2.6 and 3.0):

>>> '%s, eggs, and %s' % ('spam', 'SPAM!')          # Formatting expression (all)
'spam, eggs, and SPAM!'

>>> '{0}, eggs, and {1}'.format('spam', 'SPAM!')    # Formatting method (2.6, 3.0)
'spam, eggs, and SPAM!'

One note here: although sequence operations are generic, methods are not—although some types share some method names, string method operations generally work only on strings, and nothing else. As a rule of thumb, Python’s toolset is layered: generic operations that span multiple types show up as built-in functions or expressions (e.g., len(X), X[0]), but type-specific operations are method calls (e.g., aString.upper()). Finding the tools you need among all these categories will become more natural as you use Python more, but the next section gives a few tips you can use right now.