`urllib.request.Request` uses mutable value as default value

[EFanZh]

BPO	38654
Nosy	@stevendaprano, @serhiy-storchaka, @EFanZh
PRs	bpo-38654: Make default value immutable #17007

^{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 2019-10-31.23:11:32.416>
created_at = <Date 2019-10-31.13:17:53.658>
labels = ['invalid', 'type-feature', 'library', '3.9']
title = '`urllib.request.Request` uses mutable value as default value'
updated_at = <Date 2019-11-01.13:58:36.125>
user = 'https://github.com/EFanZh'

bugs.python.org fields:

activity = <Date 2019-11-01.13:58:36.125>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2019-10-31.23:11:32.416>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2019-10-31.13:17:53.658>
creator = 'EFanZh'
dependencies = []
files = []
hgrepos = []
issue_num = 38654
keywords = []
message_count = 7.0
messages = ['355750', '355757', '355765', '355774', '355775', '355778', '355817']
nosy_count = 3.0
nosy_names = ['steven.daprano', 'serhiy.storchaka', 'EFanZh']
pr_nums = ['17007']
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38654'
versions = ['Python 3.9']

[EFanZh]

The headers argument of the __init__ method of urllib.request.Request class uses {} as default value, which is mutable. It is not a good idea.

[serhiy-storchaka]

There is nothing wrong with using {} as default value.

[stevendaprano]

I agree with Serhiy that using mutable defaults is not automatically a bad idea. This is unnecessary code churn that fixes no bugs and adds no new functionality and doesn't make the code "better".

The PR removes one harmless use of a mutable default but adds the "code smell" that assumes that None is the only possible falsey value. This means that the __init__ method will accept any falsey value without complaint:

    Request(url, headers=0.0)

which is worse than the current code.

[EFanZh]

How about changing if headers to if headers is not None?

[serhiy-storchaka]

It is not needed. There is no bug. Nothing to change.

[EFanZh]

I agree that this doesn’t fix any bug. But is is easy to make mistakes by using mutable default values (See https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments). Both PyCharm and pylint show warnings about mutable default arguments even there is no bug in the code.

Using {} as default value, by looking at the function signature, I might wonder whether this argument stays the same on each function call, since it is mutable. But by using None as default value, I am know this argument will always be None on each function call. Using None as default value makes the function interface more easy to understand.

Also, if some new Python coders saw [] or {} being used as default values in the standard library, they might think “I’ll do it too since the standard library does it”.

[stevendaprano]

Also, if some new Python coders saw [] or {} being used as default
values in the standard library, they might think “I’ll do it too since
the standard library does it”.

Great! Having Python coders learn good progamming skills from the
standard library is a *good* thing.