argparse: wrong type from get_default when type is set

[ErikAhln]

BPO	38736
Files	test.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2019-11-13.05:57:24.705>
created_at = <Date 2019-11-07.15:40:47.721>
labels = ['3.7', 'type-feature', 'library']
title = 'argparse: wrong type from get_default when type is set'
updated_at = <Date 2019-11-13.05:57:24.704>
user = 'https://bugs.python.org/ErikAhln'

bugs.python.org fields:

activity = <Date 2019-11-13.05:57:24.704>
actor = 'paul.j3'
assignee = 'none'
closed = True
closed_date = <Date 2019-11-13.05:57:24.705>
closer = 'paul.j3'
components = ['Library (Lib)']
creation = <Date 2019-11-07.15:40:47.721>
creator = 'Erik Ahl\xc3\xa9n'
dependencies = []
files = ['48700']
hgrepos = []
issue_num = 38736
keywords = []
message_count = 4.0
messages = ['356194', '356222', '356236', '356247']
nosy_count = 2.0
nosy_names = ['paul.j3', 'Erik Ahl\xc3\xa9n']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38736'
versions = ['Python 3.7']

[ErikAhln]

The type of the object returned by get_default isn't converted to the specified type supplied to add_argument. I would expect the type to be the same.

[paulj3]

get_default just returns the default attribute as it is stored in the Action. Defaults are stored as given.

During parsing the Action's default is placed in the args namespace at the start. At the end, it checks if the value in the namespace is a string that matches the default (i.e. has been over written by user input). Only then is it passed through the type function.

[ErikAhln]

So, not a bug since you can just do default = Path('file.txt')?

[paulj3]

Yes you can set the default to be any object, such as an evaluated string. If it isn't a string it won't be passed through 'type'.

The purpose of the delayed evaluation that I described is to avoid unnecessary evaluations. The worse case would be a write filetype. You don't want to create (or over write) a default file if it never gets used.

(I intend to close this issue).