regex - match character which is not escaped

I'm trying to make a regex to match unescaped comma characters in a string.

The rule I'm looking for is "A comma not preceded by an even number of backslashes".

Test cases:

True    abc,abc
False   abc\,abc
True    abc\\,abc
False   abc\\\,abc
True    abc\\\\,abc
False   abc\\\\\,abc

I tried to use a negative look-behind: (?<!(\\+)), but Python gives me error: look-behind requires fixed-width pattern.