Consider a simple program:

import fire

def add(a, zero=0):
    print('calculating...')
    print(a + zero)

if __name__ == '__main__':
    fire.Fire(add)

And then suppose we make a typo in the argument name, writing --zerro instead of --zero. This is what I get with fire 0.1.3 under Python 3.6:

$ python t.py 1 --zerro 2
calculating...
1
Fire trace:
1. Initial component
2. Called routine "add" (t.py:3)
3. ('Could not consume arg:', '--zerro')

Type:        NoneType
String form: None

Usage:       t.py 1 -

Notice that first we run the code, and only then the error is reported. While I expected the errors to be checked before any user code is executed, because this code could be working for a long time, doing wrong things, etc.