[ Python 常見問題 ] Parsing boolean values with argparse

Source From Here 
Question 
I would like to use argparse to parse boolean command-line arguments written as "--foo True" or "--foo False". For example: 

# my_program --my_boolean_flag False

However, the following test code does not do what I would like: 

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1. import argparse  
2. parser = argparse.ArgumentParser(description="My parser")  
3. parser.add_argument("--my_bool", type=bool)  
4. cmd_line = ["--my_bool", "False"]  
5. parsed_args = parser.parse(cmd_line)  

Sadly, parsed_args.my_bool evaluates to True. This is the case even when I change cmd_line to be ["--my_bool", ""], which is surprising, since bool("") evalutates to False. How can I get argparse to parse "False", "F", and their lower-case variants to be False? 

How-To 
You can define a API to handle/translate the input into True/False: 

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1. def str2bool(v):  
2.     if v.lower() in ('yes', 'true', 't', 'y', '1'):  
3.         return True  
4.     elif v.lower() in ('no', 'false', 'f', 'n', '0'):  
5.         return False  
6.     else:  
7.         raise argparse.ArgumentTypeError('Boolean value expected.')  

The full example code: 
- test.py 

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1. import argparse  
2.   
3. def str2bool(v):  
4.     if v.lower() in ('yes', 'true', 't', 'y', '1'):  
5.         return True  
6.     elif v.lower() in ('no', 'false', 'f', 'n', '0'):  
7.         return False  
8.     else:  
9.         raise argparse.ArgumentTypeError('Boolean value expected.')  
10.   
11. parser = argparse.ArgumentParser(description='For testing')  
12. parser.add_argument("--nice", "-n", type=str2bool, nargs='?',  
13.                         const=True, default=True, required=True,  
14.                         help="Activate nice mode.")  
15. args = parser.parse_args()  
16.   
17. print("Nice=%s" % (args.nice))  

Then test it this way: 

# ./test.py 
usage: test.py [-h] --nice [NICE] 
test.py: error: argument --nice/-n is required 
# ./test.py -n t 
Nice=True 
# ./test.py -n f 
Nice=False 
# ./test.py -n True 
Nice=True