There is a possibility that someone (like me) accidentally will omit parentheses with FileType arguments after FileType, and parser will contain wrong file until someone will try to use it. 

Example:
parser = argparse.ArgumentParser()
parser.add_argument('-x', type=argparse.FileType)

I don't see an easy way around this.  FileType is a class, and thus is a callable.  add_argument checks that the 'type' parameter is a callable (or a string in the registry).  Otherwise it gives the programmer a lot of freedom regarding this parameter.

Reading the examples in the doc, it's clear the behavior when FileType takes an argument. What's the behavior of FileType when is called without any argument?

The docs specify what argparse.FileType() does via the default parameters: https://docs.python.org/3/library/argparse.html#argparse.FileType

If you mean what does it do when you fail to call it at all (passing type=argparse.FileType), the answer is the same as any other class: It tries to construct a FileType using the user provided argument as the sole positional argument. So if the user passes a valid mode string, it works, and returns a FileType object with that mode string, otherwise it barfs and dumps an error message for passing an invalid argument for FileType.

Yes, I meant that. Thanks! :)

If you will run `python test.py hello.txt, where test.py is:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('echo', type=argparse.FileType)
args = parser.parse_args()
print(args.echo)

You will receive:
FileType('hello.txt')

I think that can be confusing for someone who will forget to invoke FileType constructor.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Tuesday, June 4, 2019 10:27 PM, Michele Angrisano <report@bugs.python.org> wrote:

>
>
> Michele Angrisanomichele.angrisano@gmail.com added the comment:
>
> Reading the examples in the doc, it's clear the behavior when FileType takes an argument. What's the behavior of FileType when is called without any argument?
>
> -----------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> nosy: +mangrisano
>
> Python tracker report@bugs.python.org
> https://bugs.python.org/issue37150

Ah, right. It doesn't actually validate the mode string (it just stores it for when open is called, assuming open will validate it). So yeah, it silently accepts any string, not just valid mode strings. Not a contractual guarantee or anything, just how FileType.__init__ is implemented.

What if I will add mode check in FileType.__init__?

New changeset 03d5831a2d62c68654ec223168e574cd546efbf6 by Miss Islington (bot) (zygocephalus) in branch 'master':
bpo-37150: Throw ValueError if FileType class object was passed in add_argument (GH-13805)
https://github.com/python/cpython/commit/03d5831a2d62c68654ec223168e574cd546efbf6

New changeset 606ac581e2451c420117c55632f0fe13d4cec2cd by Miss Islington (bot) in branch '3.8':
bpo-37150: Throw ValueError if FileType class object was passed in add_argument (GH-13805)
https://github.com/python/cpython/commit/606ac581e2451c420117c55632f0fe13d4cec2cd