第821页 | Learning Python | 阅读 ‧ 电子书库

Method Calls

If you really want to see what attributes str has that bytes doesn’t, you can always check their dir built-in function results. The output can also tell you something about the expression operators they support (e.g., __mod__ and __rmod__ implement the % operator):

C:\misc> c:\python30\python

# Attributes unique to str

>>> set(dir('abc')) - set(dir(b'abc'))
{'isprintable', 'format', '__mod__', 'encode', 'isidentifier',
'_formatter_field_name_split', 'isnumeric', '__rmod__', 'isdecimal',
'_formatter_parser', 'maketrans'}

# Attributes unique to bytes

>>> set(dir(b'abc')) - set(dir('abc'))
{'decode', 'fromhex'}

As you can see, str and bytes have almost identical functionality. Their unique attributes are generally methods that don’t apply to the other; for instance, decode translates a raw bytes into its str representation, and encode translates a string into its raw bytes representation. Most of the methods are the same, though bytes methods require bytes arguments (again, 3.0 string types don’t mix). Also recall that bytes objects are immutable, just like str objects in both 2.6 and 3.0 (error messages here have been shortened for brevity):

>>> B = b'spam'                    # b'...' bytes literal
>>> B.find(b'pa')
1

>>> B.replace(b'pa', b'XY')        # bytes methods expect bytes arguments
b'sXYm'

>>> B.split(b'pa')
[b's', b'm']

>>> B
b'spam'

>>> B[0] = 'x'
TypeError: 'bytes' object does not support item assignment

One notable difference is that string formatting works only on str objects in 3.0, not on bytes objects (see Chapter 7 for more on string formatting expressions and methods):

>>> b'%s' % 99
TypeError: unsupported operand type(s) for %: 'bytes' and 'int'

>>> '%s' % 99
'99'

>>> b'{0}'.format(99)
AttributeError: 'bytes' object has no attribute 'format'

>>> '{0}'.format(99)
'99'