Mixing String Types

In the replace call of the section Method Calls, we had to pass in two bytes objects—str types won’t work there. Although Python 2.X automatically converts str to and from unicode when possible (i.e., when the str is 7-bit ASCII text), Python 3.0 requires specific string types in some contexts and expects manual conversions if needed:

# Must pass expected types to function and method calls

>>> B = b'spam'

>>> B.replace('pa', 'XY')
TypeError: expected an object with the buffer interface

>>> B.replace(b'pa', b'XY')
b'sXYm'

>>> B = B'spam'
>>> B.replace(bytes('pa'), bytes('xy'))
TypeError: string argument without an encoding

>>> B.replace(bytes('pa', 'ascii'), bytes('xy', 'utf-8'))
b'sxym'


# Must convert manually in mixed-type expressions

>>> b'ab' + 'cd'
TypeError: can't concat bytes to str

>>> b'ab'.decode() + 'cd'                   # bytes to str
'abcd'

>>> b'ab' + 'cd'.encode()                   # str to bytes
b'abcd'

>>> b'ab' + bytes('cd', 'ascii')            # str to bytes
b'abcd'

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Although you can create bytes objects yourself to represent packed binary data, they can also be made automatically by reading files opened in binary mode, as we’ll see in more detail later in this chapter. First, though, we should introduce bytes’s very close, and mutable, cousin.