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?
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