Tested (on windows) with python 3.6, but I guess it's the same in py3.7: 

# test.py
def hello(x):
  print(_(f'hello {x}'))



> py pygettext.py test.py
Traceback (most recent call last):
  File "C:\Program Files\Python36\Tools\i18n\pygettext.py", line 623, in <module>
    if __name__ == '__main__':
  File "C:\Program Files\Python36\Tools\i18n\pygettext.py", line 597, in main
    for _token in tokens:
  File "C:\Program Files\Python36\Tools\i18n\pygettext.py", line 328, in __call__
    ##              'tstring:', tstring
  File "C:\Program Files\Python36\Tools\i18n\pygettext.py", line 382, in __openseen
    elif ttype == tokenize.STRING:
  File "C:\Program Files\Python36\Tools\i18n\pygettext.py", line 236, in safe_eval
    # unwrap quotes, safely
  File "<string>", line 1, in <module>
NameError: name 'x' is not defined

Is there really a use case for this?

I would normally expect place holders to get replaced *after* translation, i.e. in the translated text, not before.

I agree that pygettext should not and can not support f-strings, but the error should be better. The problem is that an f-string expression is tokenized as literal strings, but eval() can't evaluate it out of context.

> the error should be better

Yes, that's what I maeant - sorry I should have phrased better. 
I marked by mistake a couple of f-strings for translation and totally forgot about it for a while... it took me some time to figure out what was going on with this NameError popping out of nowhere.

Curiously, xgettext handles this thing better (...or, far worse actually): it just parses the f-string as a regular one, producing something like
msgid "hello {x}"
msgstr ""
But then of course Python will fail *silently* at runtime, never picking up the translated version of the string...

If there are no objections I'm going to merge PR 6364 in 24 hours.

New changeset 69524821a87251b7aee966f6e46b3810ff5aaa64 by Serhiy Storchaka in branch 'master':
bpo-33189: pygettext.py now accepts only literal strings (GH-6364)
https://github.com/python/cpython/commit/69524821a87251b7aee966f6e46b3810ff5aaa64

New changeset a4fb580f701df5bf07ce569a4f43abfb05c92759 by Miss Islington (bot) in branch '3.7':
bpo-33189: pygettext.py now accepts only literal strings (GH-6364)
https://github.com/python/cpython/commit/a4fb580f701df5bf07ce569a4f43abfb05c92759

New changeset 6f870935c2e024cbd1cc379f85e6a66d7711dcc7 by Miss Islington (bot) in branch '3.6':
bpo-33189: pygettext.py now accepts only literal strings (GH-6364)
https://github.com/python/cpython/commit/6f870935c2e024cbd1cc379f85e6a66d7711dcc7