f-string conversion with advanced string formatting & None

[peterjc]

Failing test case using pyupgrade 2.25.0:

$ echo 'a=b=c=None; print("Query:%8s %s %s" % (a, b, c))'  > test.py ; pyupgrade --py36-plus test.py ; cat test.py
Rewriting test.py
a=b=c=None; print(f"Query:{a:>8} {b} {c}")

>>> a=b=c=None; print("Query:%8s %s %s" % (a, b, c))
Query:    None None None
>>> a=b=c=None; print(f"Query:{a:>8} {b} {c}")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to NoneType.__format__

The problem is while f"{None}" works, advanced string format arguments do not:

>>> "%8s" % None
'    None'

>>> f"{None:>8}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to NoneType.__format__

Possible workaround:

>>> a=b=c=None; print(f"Query:{str(a):>8} {b} {c}")
Query:    None None None

This issue was found in biopython/biopython#3724

[asottile]

sadly I think this is unfixable -- there would need to be type analysis to properly know that "None" is buggy -- the only other option is disabling all alignment conversions

arguably this is a bug in python itself, I see no reason that the padding formatters shouldn't work for None

I'm not sure what the best thing to do here is

:8 is more concise but incorrect:

>>> '{:8}'.format("a")
'a       '
>>> '{:8}'.format(1)
'       1'
>>> '%8s' % ('a',)
'       a'
>>> '%8s' % (1,)
'       1'

an aside, please don't mention flynt -- the author has ripped me off in the past and has generally been an uncooperative and unethical open source maintainer

[asottile]

I guess the only safe approach is skipping -- #529 implements that

[peterjc]

I was also wondering if this was a Python bug too, I had a quick look but didn't find anything on their tracker.

I agree that unless a future version of Python changes this, disabling any conversion with an alignment is safest.

[asottile]

we'll see where this goes: https://bugs.python.org/issue45165