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