• Choose a category
• All
• Classic-Fiction

By Root 1477 0

different things for different objects: addition for numbers, and concatenation for strings. This is a general property of Python that we’ll call polymorphism later in the book—in sum, the meaning of an operation depends on the objects being operated on. As you’ll see when we study dynamic typing, this polymorphism property accounts for much of the conciseness and flexibility of Python code. Because types aren’t constrained, a Python-coded operation can normally work on many different types of objects automatically, as long as they support a compatible interface (like the + operation here). This turns out to be a huge idea in Python; you’ll learn more about it later on our tour.

Immutability

Notice that in the prior examples, we were not changing the original string with any of the operations we ran on it. Every string operation is defined to produce a new string as its result, because strings are immutable in Python—they cannot be changed in-place after they are created. For example, you can’t change a string by assigning to one of its positions, but you can always build a new one and assign it to the same name. Because Python cleans up old objects as you go (as you’ll see later), this isn’t as inefficient as it may sound:

>>> S

'Spam'

>>> S[0] = 'z' # Immutable objects cannot be changed

...error text omitted...

TypeError: 'str' object does not support item assignment

>>> S = 'z' + S[1:] # But we can run expressions to make new objects

>>> S

'zpam'

Every object in Python is classified as either immutable (unchangeable) or not. In terms of the core types, numbers, strings, and tuples are immutable; lists and dictionaries are not (they can be changed in-place freely). Among other things, immutability can be used to guarantee that an object remains constant throughout your program.

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:

>>> 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