[ Python 常見問題 ] Require either of two arguments using argparse

Source From Here 
Question 
Given: 

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1. import argparse  
2.   
3. pa = argparse.ArgumentParser()  
4. pa.add_argument('--foo')  
5. pa.add_argument('--bar')  

How do I make at most one of "foo, bar" mandatory: --foo x or --bar y are fine, --foo x --bar y is not? 

How-To 
I think you are searching for something like mutual exclusion. This way, only foo or bar will be accepted, not both: 
* ArgumentParser.add_mutually_exclusive_group(required=False): 

Create a mutually exclusive group. argparse will make sure that only one of the arguments in the mutually exclusive group was present on the command line

For example: 
- test.py 

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1. import argparse  
2.   
3. import argparse  
4.   
5. parser = argparse.ArgumentParser(description='For testing')  
6. group = parser.add_mutually_exclusive_group(required=True)  
7. group.add_argument('--foo', "-f",action='store_true', default=False)  
8. group.add_argument('--bar', "-b",action='store_true', default=False)  
9. args = parser.parse_args()  
10.   
11. print("foo=%s; bar=%s" % (args.foo, args.bar))  

Then test it this way: 

# ./test.py 
usage: test.py [-h] (--foo | --bar) 
test.py: error: one of the arguments --foo/-f --bar/-b is required 
# ./test.py -f 
foo=True; bar=False 
# ./test.py -b 
foo=False; bar=True