argparse fails when called from unittest test

Question

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…

Accepted Answer

`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")
```

… because you'll get `'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.

argparse fails when called from unittest test

Solution

Problem