Can you make Python3 give an error when comparing strings to bytes

Question

When converting code from Python 2 to Python 3 one issue is that the behaviour when testing strings and bytes for equality has changed. For example:

foo = b'foo'
if foo == 'foo':
    print("They match!")

prints nothing on Python 3 and "They match!" on Python 2. In this case it is easy to spot but in many cases the check is performed on variables which may have been defined elsewhere so there is no obvious type information.

I would like to make the Python 3 interpreter give an error whenever there is an equality test between string and bytes rather than silently conclude that they are different. Is there any way to accomplish this?

Answer 1

(EDITED: to fix an issue where I was incorrectly suggesting that modifying __eq__ on the instance would affect the == evaluation as suggested by @user2357112supportsMonica).

Normally, you would do this by overriding the __eq__ method of the type(s) you would like to guard. Unfortunately for you, this cannot be done for built-in types, notably str and bytes, therefore code like this:

foo = b'foo'
bytes.__eq__ = ...  # a custom equal function
# str.__eq__ = ...  # if it were 'foo' == foo (or `type(foo)`)
if foo == 'foo':
    print("They match!")

would just throw:

AttributeError: 'bytes' object attribute '__eq__' is read-only

You may need to manually guard the comparison with something like:

def str_eq_bytes(x, y):
    if isinstance(x, str) and isinstance(y, bytes):
        raise TypeError("Comparison between `str` and `bytes` detected.")
    elif isinstance(x, bytes) and isinstance(y, str):
        raise TypeError("Comparison between `bytes` and `str` detected.")

to be used as follows:

foo = 'foo'
if str_eq_bytes(foo, 'foo') or foo == 'foo':
    print("They match!")
# They match!

foo = 'bar'
if str_eq_bytes(foo, 'foo') or foo == 'foo':
    print("They match!")
# <nothing gets printed>

foo = b'foo'
if str_eq_bytes(foo, 'foo') or foo == 'foo':
    print("They match!")

TypeError: Comparison between `bytes` and `str` detected.

The other option would be to hack in your own Python fork and override __eq__. Note that also Pypy does not allow you to override methods for built-in types.

Answer 2

There is an option, -b, you can pass to the Python interpreter to cause it to emit a warning or error when comparing byte / str.

> python --help
usage: /bin/python [option] ... [-c cmd | -m mod | file | -] [arg] ...
Options and arguments (and corresponding environment variables):
-b     : issue warnings about str(bytes_instance), str(bytearray_instance)
         and comparing bytes/bytearray with str. (-bb: issue errors)

This produces a BytesWarning as seen here:

> python -bb -i
Python 3.8.0
Type "help", "copyright", "credits" or "license" for more information.
>>> v1 = b'foo'
>>> v2 = 'foo'
>>> v1 == v2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
BytesWarning: Comparison between bytes and string