[issue24754] argparse add_argument with action="store_true", type=bool should not crash

[Joannah Nanjekye]

Joannah Nanjekye  added the comment:

See issue25299 also.

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nosy: +nanjekyejoannah

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[issue24754] argparse add_argument with action=store_true, type=bool should not crash

[Konstantin Molchanov]

Konstantin Molchanov added the comment:

Although I agree that specifying type with store_true or store_false is 
unnecessary, this shouldn't really be an error like this. Why not just ignore 
type if it can't be utilized?

The error message implies the usage of add_argument is erroneous, however it is 
fully compatible with the spec give in the docs.

Alternatively, the docs should be updated.

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nosy: +moigagoo

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[issue24754] argparse add_argument with action=store_true, type=bool should not crash

[R. David Murray]

Changes by R. David Murray rdmur...@bitdance.com:


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stage:  - resolved

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[issue24754] argparse add_argument with action=store_true, type=bool should not crash

[Douglas Bagnall]

Douglas Bagnall added the comment:

Thanks paul.j3 and r.david.murray,

I see your points.

What led me to this was automatically generating command line options from a 
Gstreamer element, mapping
the Gstreamer property types to Python types. Then I special-cased the 
action=store_true for the boolean ones and was surprised by the somewhat 
unclear error message.

As you suggest, paul.j3, type is in some ways an unfortunate choice of 
keyword for this, but it too late to change that.

--
resolution:  - not a bug
status: open - closed

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[issue24754] argparse add_argument with action=store_true, type=bool should not crash

[paul j3]

paul j3 added the comment:

Right, the only arguments that a `store_true` Action class accepts are:

 option_strings, dest, default=False,required=False, help=None

parser.add_argument() massages its parameters a bit, and then uses the 'action' 
parameter to construct an Action subclass object with the other parameters.  
Depending on their needs the subclasses may set various defaults or omit 
certain parameters. And without a catchall '**kwargs', the result of providing 
an omitted parameter is this error message.

For 'store_true', a 'type' parameter is superfluous.  It doesn't take any 
arguments (it sets 'nargs=0'), so there aren't any arguments to convert with 
the 'type' function.  It's the Action's own 'default' and 'const' that provide 
the required False and True values.

Also 'bool' is not a meaningful 'type' - for any Action class.  There isn't a 
function in Python converts a string to a boolean.  'bool()' does not do this.  
Or rather, `bool(astr)' converts every string to True except the empty one.

I've seen this mistake before, assuming that 'type=bool' means 'I want the 
result to be a boolean'.  But the docs do say 

type= can take any callable that takes a single string argument and returns 
the converted value:

'type=float' means 'use float(astr) to convert astr', not 'the argument should 
be a float'.

The documentation is not clear about what parameters are valid for each Action 
type.  It indicates, in bits and pieces, that some parameters are meaningful 
for certain actions, and not useful for others.  One (version) even takes a 
parameter that others don't.  But I haven't seen many bugs - here or on 
StackOverflow - where this is an issue.

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[issue24754] argparse add_argument with action=store_true, type=bool should not crash

[R. David Murray]

R. David Murray added the comment:

Looking at the source it appears that there are many actions for which it is 
not legal to also specify type.  That is, this looks like a design decision 
rather than a bug.

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[issue24754] argparse add_argument with action=store_true, type=bool should not crash

[Douglas Bagnall]

New submission from Douglas Bagnall:

A line like this:

parser.add_argument('--hello', action=store_true, type=bool)

causes a TypeError in 2.7 and 3.4:

   File /usr/lib/python3.4/argparse.py, line 1344, in add_argument
 action = action_class(**kwargs)

 TypeError: __init__() got an unexpected keyword argument 'type'

It shouldn't really.

--
components: Library (Lib)
files: argparse-crash.py
messages: 247658
nosy: dbagnall
priority: normal
severity: normal
status: open
title: argparse add_argument with action=store_true, type=bool should not 
crash
versions: Python 2.7, Python 3.4
Added file: http://bugs.python.org/file40065/argparse-crash.py

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