[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Raymond Hettinger]

Change by Raymond Hettinger :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset facca72eae3c0b59b4e89bab81c936dff10fb58a by Miss Islington (bot) 
in branch '3.9':
bpo-38843: Document behavior of default when the attribute is already set 
(GH-23653) (#23668)
https://github.com/python/cpython/commit/facca72eae3c0b59b4e89bab81c936dff10fb58a


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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset 752cdf21eb2be0a26ea6a34a0de33a458459aead by Raymond Hettinger in 
branch 'master':
bpo-38843: Document behavior of default when the attribute is already set 
(GH-23653)
https://github.com/python/cpython/commit/752cdf21eb2be0a26ea6a34a0de33a458459aead


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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[miss-islington]

Change by miss-islington :


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nosy: +miss-islington
nosy_count: 4.0 -> 5.0
pull_requests: +22535
pull_request: https://github.com/python/cpython/pull/23668

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Raymond Hettinger]

Change by Raymond Hettinger :


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keywords: +patch
pull_requests: +22521
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23653

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee: docs@python -> rhettinger

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Ivan, you don't need to specify default values to have typing.  This will 
suffice:

class CliArgs(object):
foo: Optional[str]
bar: int
baz: float

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Raymond Hettinger]

Raymond Hettinger  added the comment:

For now, we should at least document that, "If the preexisting namespace has an 
attribute set, the action default will not over write it."

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Ivan Kurnosov]

Ivan Kurnosov  added the comment:

> I have not seen many questions about the use of a preexisting namespace 
> object (here or on StackOverflow)

as typing was added to the language natively - it should become more and more 
frequently used.

I personally see no reason anymore to NOT use a custom namespace: typed 
arguments object is x100 times better than untyped one.

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[paul j3]

paul j3  added the comment:

It doesn't have to be a special class. It can be a `argparse.Namespace` object. 
 If the preexisting namespace has an attribute set, the action default will not 
over write it.

In [85]: parser = argparse.ArgumentParser() 
...: parser.add_argument('--foo', default='bar') 
...: parser.parse_args([],
 namespace=argparse.Namespace(foo=123, baz=132))   
Out[85]: Namespace(baz=132, foo=123)

This is described in comments at the start of parse_known_args()


# add any action defaults that aren't present
for action in self._actions:
if action.dest is not SUPPRESS:
if not hasattr(namespace, action.dest):
if action.default is not SUPPRESS:
setattr(namespace, action.dest, action.default)

# add any parser defaults that aren't present
for dest in self._defaults:
if not hasattr(namespace, dest):
setattr(namespace, dest, self._defaults[dest])

There are many details about 'defaults' that are not documented.  This might 
not be the most significant omission.  

I have not seen many questions about the use of a preexisting namespace object 
(here or on StackOverflow).  While such a namespace can be used to set custom 
defaults (as shown here), I think it is more useful when using a custom 
Namespace class, one the defines special behavior.

Originally the main parser's namespace was passed to subparsers.  But a change 
in 2014, gave the subparser a fresh namespace, and then copied values from it 
back to the main namespace.  While that gave more power to the subparser's 
defaults, users lost some ability to use their own namespace class.

https://bugs.python.org/issue27859 - argparse - subparsers does not retain 
namespace

https://bugs.python.org/issue9351 - argparse set_defaults on subcommands should 
override top level set_defaults

https://bugs.python.org/issue34827 - Make argparse.NameSpace iterable (closed)

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +paul.j3, rhettinger

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[issue38843] Document argparse behaviour when custom namespace object already has the field set

[Ivan Kurnosov]

New submission from Ivan Kurnosov :

At this moment it's impossible to explain the behaviour of this script using 
documentation.

Given it was explicitly coded to behave like that - it should be somehow noted 
in the documentation, that as long as a `CliArgs.foo` field has a default value 
set already - it won't be overwritten with a default argparse argument value.


```
import argparse

class CliArgs(object):
foo: str = 'not touched'


parser = argparse.ArgumentParser()
parser.add_argument('--foo', default='bar')

args = CliArgs()
parser.parse_args(namespace=args)
print(args.foo) # 'not touched'

print(parser.parse_args()) # 'bar'
```

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assignee: docs@python
components: Documentation
messages: 356939
nosy: docs@python, zerkms
priority: normal
severity: normal
status: open
title: Document argparse behaviour when custom namespace object already has the 
field set

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