unexpected behavior for booleans in argparse

[NathanNaze]

BPO	26994
Nosy	@bitdancer
Superseder	bpo-21208: Change default behavior of arguments with type bool when options are specified

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

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GitHub fields:

assignee = None
closed_at = <Date 2016-05-10.18:37:48.266>
created_at = <Date 2016-05-10.18:30:29.946>
labels = ['type-bug', 'library']
title = 'unexpected behavior for booleans in argparse'
updated_at = <Date 2016-05-14.17:00:50.659>
user = 'https://bugs.python.org/NathanNaze'

bugs.python.org fields:

activity = <Date 2016-05-14.17:00:50.659>
actor = 'paul.j3'
assignee = 'none'
closed = True
closed_date = <Date 2016-05-10.18:37:48.266>
closer = 'r.david.murray'
components = ['Library (Lib)']
creation = <Date 2016-05-10.18:30:29.946>
creator = 'Nathan Naze'
dependencies = []
files = []
hgrepos = []
issue_num = 26994
keywords = []
message_count = 6.0
messages = ['265256', '265257', '265259', '265260', '265284', '265535']
nosy_count = 3.0
nosy_names = ['r.david.murray', 'paul.j3', 'Nathan Naze']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '21208'
type = 'behavior'
url = 'https://bugs.python.org/issue26994'
versions = ['Python 2.7']

[NathanNaze]

Setting a boolean type in argparse gives unexpected behavior when setting "True", "False", etc.

https://gist.github.com/nanaze/db63e3f63e318408e3223bf1245d9752

Would have expected parsing to fail for unclear input that doesn't neatly map to a boolean value.

[bitdancer]

type=bool doesn't do what you think it does.  It does 'bool(value)', and 'bool("False")' is True, since "False" is a non-empty string.

See bpo-21208 for some further discussion.

[NathanNaze]

It does 'bool(value)', and 'bool("False")' is True, since "False" is a non-empty string.

Yes, I understand this. It's fine to mark as "working as intended", but coming from other flag-parsing libraries, I find the behavior unintuitive and do not understand the utility of accepting arbitrary strings given the potential for user confusion. We uncovered this behavior debugging a script used internally at Google.

[NathanNaze]

I also buy the argument that changing the behavior now would be problematic given the existing usages in the wild.

[bitdancer]

Yes, it is an unitended consequence of the fact that argparse types are arbitrary single argument functions (that take an arbitrary string as the argument and convert it), and bool is a single argument function. Unfortunately we're stuck with it now. The correct answer, of course, is to write your own bool type converter if you need one.

It is possible there should be a documentation mention that bool is not approprate as a type function, since int and float, for example, are explicitly mentioned. If you want to submit a patch to that end I'll reopen the issue.

[paulj3]

I answered a similar question recently on Stackoverflow when the user wanted to use type=hex.

http://stackoverflow.com/questions/37006387/python-argparse-hex-error

In another recent bug/issue the poster want a enum type. It's not hard to define a function, or factory class, that handles mappings like this, but it isn't as simple or intuitive as some would like.

There is a registries mechanism, which could allow the user to use type='bool', where 'bool' is the name of a function that that converts some set of strings to True/False. But people are used to providing strings for the action, they aren't used to do so for the type.

http://stackoverflow.com/questions/15008758/parsing-boolean-values-with-argparse