grp.getgrgid is capable of accepting a string:

from grp import getgrgid
print(getgrgid('0'))

However, pwd.getpwuid can't do the same:

from pwd import getpwuid
print(getpwuid('0'))

Traceback (most recent call last):
  File "getpwuid_test.py", line 2, in <module>
    print(getpwuid('0'))
TypeError: an integer is required

This seems to be because inside Modules/pwdmodule.c, getpwuid uses PyNumber_ParseTuple with a converter that uses PyNumber_Index to get a Python integer, and that raises an exception on failure.

However, in Modules/grpmodule.c, grp_getgrgid uses PyNumber_Long (Or PyNumber_Int for an old enough Python) as a conversion first, and as the documentation says at https://docs.python.org/3/c-api/number.html, this is the equivalent of running int(o), which can convert a string to an integer. Only then is it given to PyNumber_Index, by way of a helper function _Py_Gid_Converter

Should these have different behaviours? Is there a reason for the difference?

The behaviour of getgrgid seems more helpful, and it's odd that it doesn't apply to both functions. Is this undesirable behaviour in getgrgid or getpwuid?