argparse fails when called from unittest test

问题:

In a file (say parser.py) I have:

import argparse  def parse_cmdline(cmdline=None):     parser = argparse.ArgumentParser()     parser.add_argument('--first-param',help="Does foo.")     parser.add_argument('--second-param',help="Does bar.")      if cmdline is not None:         args = parser.parse_args(cmdline)     else:         args = parser.parse_args()      return vars(args)  if __name__=='__main__':     print parse_cmdline() 

Sure enough, when called from the command line it works and give me pretty much what I expect:

$ ./parser.py --first-param 123 --second-param 456 {'first_param': '123', 'second_param': '456'} 

But then I want to unittest it, thus I write a test_parser.py file:

import unittest from parser import parse_cmdline  class TestParser(unittest.TestCase):     def test_parse_cmdline(self):         parsed = parse_cmdline("--first-param 123 --second-param 456")          self.assertEqual(parsed['first_param'],'123')         self.assertEqual(parsed['second_param'],'456')  if __name__ == '__main__':     unittest.main() 

Then I get the following error:

usage: test_parser.py [-h] [--first-param FIRST_PARAM]                       [--second-param SECOND_PARAM] test_parser.py: error: unrecognized arguments: - - f i r s t - p a r a m   1 2 3   - - s e c o n d - p a r a m   4 5 6 E ====================================================================== ERROR: test_parse_cmdline (__main__.TestParser) ---------------------------------------------------------------------- Traceback (most recent call last):   File "./test_parser.py", line 8, in test_parse_cmdline     parsed = parse_cmdline("--first-param 123 --second-param 456")   File "/home/renan/test_argparse/parser.py", line 12, in parse_cmdline     args = parser.parse_args(cmdline)   File "/usr/lib/python2.7/argparse.py", line 1691, in parse_args     self.error(msg % ' '.join(argv))   File "/usr/lib/python2.7/argparse.py", line 2361, in error     self.exit(2, _('%s: error: %s\n') % (self.prog, message))   File "/usr/lib/python2.7/argparse.py", line 2349, in exit     _sys.exit(status) SystemExit: 2  ---------------------------------------------------------------------- Ran 1 test in 0.004s  FAILED (errors=1) 

As can be seen, the command line I specified (--first-param 123 --second-param 456) became - - f i r s t - p a r a m 1 2 3 - - s e c o n d - p a r a m 4 5 6 (each character is separated by a space).

I don't understand why: what am I doing wrong?

回答1:

argparse wants an "argument vector"―that is, a list of separate arguments―not a "command line" string.

And just calling split won't solve things. For example, this command line from the shell:

python script.py --first-param '123 456' --second-param 789 

… will correctly give you 123 456 and 789, but this line of code:

parse_cmdline("--first-param '123 456' --second-param 789") 

will not; it will give you '123 and 789, with an extra 456' that it doesn't know how to deal with.

In fact, even this is wrong:

parse_cmdline("--first-param '123' --second-param 789") 

'123' instead of 123.

---

There are two ways to deal with this.

First, if you know what you're trying to pass, you can just pass it as a list instead of a string in the first place, and you don't need to worry about fiddly quoting and splitting details:

parse_cmdline(["--first-param", "123 456", "--second-param", "789"]) 

Alternatively, if you don't know exactly what you're trying to pass, you just know what it looks like on the shell, you can use shlex:

if cmdline is not None:     args = parser.parse_args(shlex.split(cmdline)) 

… and now Python will split your command line the same way as a standard Unix shell, getting the 123 456 as a single parameter.

回答2:

To answer myself (I realized my mistake a few minutes later):

I am supposed to

if cmdline is not None:     args = parser.parse_args(cmdline.split()) else:     args = parser.parse_args() 

Now the test passes correctly!

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