Professional Python by Sneeringer Luke

Author:Sneeringer, Luke [Sneeringer, Luke]
Language: eng
Format: azw3, epub
ISBN: 9781119070788
Publisher: Wiley
Published: 2015-10-06T16:00:00+00:00

Python 2 Strings

Python 2 strings mostly work similarly, but with some subtle but very important distinctions.

The first distinction is the name of the classes. The Python 3 str class is called unicode in Python 2. In and of itself, this is fine. However, the Python 3 bytes class is called str in Python 2. This means that a Python 3 str is a text string, whereas a Python 2 str is a byte string. If you are using Python 2, it is critically important to understand this distinction.

Instantiating a string with no prefix gives you a str (remember, this is a byte string!) instance.

>>> byte_str = 'The quick brown fox jumped over the lazy dogs.' >>> type(byte_str) <type 'str'>

If you want a text string in Python 2, you prefix the string literal with the u character, as shown here:

>>> text_str = u'The quick brown fox jumped over the lazy dogs.' >>> type(text_str) <type 'unicode'>

Unlike Python 3, Python 2 does attempt to implicitly convert between text strings and byte strings. The way that this works is that if the interpreter encounters a mixed operation, it will first convert the byte string to a text string, and then perform the operation against the text strings.

It works this way so that an operation against a byte string and a text string will return a text string:

>>> 'foo' + u'bar' u'foobar'

The interpreter performs this implicit decoding using whatever the default encoding is. On Python 2, this is almost always ASCII. Python defines a method, sys.getdefaultencoding, which provides the default codec for implicitly converting between text strings and byte strings.

>>> import sys >>> sys.getdefaultencoding() 'ascii'

This means that many of the previous Python 3 examples show distinctly different behavior in Python 2.

>>> 'foo' == u'foo' True >>> >>> d = {u'foo': u'bar'} >>> d['foo'] u'bar'

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