Why in argparse, a 'True' is always 'True'? [duplicate]

Question

Here is the simplest Python script, named test.py:

import argparse  parser = argparse.ArgumentParser() parser.add_argument('--bool', default=True, type=bool, help='Bool type') args = parser.parse_args() print(args.bool) 

But when I run this code on the command line:

python test.py --bool False True 

Whereas when my code reads '--bool', default=False, the argparse runs correctly.

Why?

Answer 1

You are not passing in the False object. You are passing in the 'False' string, and that's a string of non-zero length.

Only a string of length 0 tests as false:

>>> bool('') False >>> bool('Any other string is True') True >>> bool('False')  # this includes the string 'False' True 

Use a store_true or store_false action instead. For default=True, use store_false:

parser.add_argument('--bool', default=True, action='store_false', help='Bool type') 

Now omitting the switch sets args.bool to True, using --bool (with no further argument) sets args.bool to False:

python test.py True  python test.py --bool False 

If you must parse a string with True or False in it, you'll have to do so explicitly:

def boolean_string(s):     if s not in {'False', 'True'}:         raise ValueError('Not a valid boolean string')     return s == 'True' 

and use that as the conversion argument:

parser.add_argument('--bool', default=True, type=boolean_string, help='Bool type') 

at which point --bool False will work as you expect it to.