UnicodeWarning fired only once

internals, python, unicode

Solution

The default configuration for Python is to use the `default` warning configuration (with a few specific exceptions). This means that warnings are emitted just once per warning, module and line.

See the `-W arg` command line switch:

By default, each warning is printed once for each source line where it occurs.

The `UnicodeWarning` will be emitted again if the same comparison problem arises in a different module or line number.

In the interactive interpreter, every time you enter some code that block starts with line number 1 again, and are always executed in the `__main__` module. So the second time you ran the comparison, the code ran as line 1 in the `__main__` module again, so the warning was suppressed.

If you triggered the issue in a different module, or on different lines, you'd see the warning again:

>>> 'ab\xDF' == u'abc'
__main__:1: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
False
>>> 'ab\xDF' == u'abc'
False
>>> if True:
...     'ab\xDF' == u'abc'
... 
__main__:2: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
False

The second comparison was on line 1 again, so the warning was suppressed, but by adding `if True:` before the comparison we got two lines and the warning was emitted again.

Problem

Original source