Python should warn when a global/local has the same name as a builtin

[reuven]

BPO	39225
Nosy	@brettcannon, @ericvsmith, @reuven

^{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 2020-01-06.18:18:32.763>
created_at = <Date 2020-01-05.22:18:09.116>
labels = ['interpreter-core']
title = 'Python should warn when a global/local has the same name as a builtin'
updated_at = <Date 2020-01-06.18:18:32.755>
user = 'https://github.com/reuven'

bugs.python.org fields:

activity = <Date 2020-01-06.18:18:32.755>
actor = 'brett.cannon'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-06.18:18:32.763>
closer = 'brett.cannon'
components = ['Interpreter Core']
creation = <Date 2020-01-05.22:18:09.116>
creator = 'reuven'
dependencies = []
files = []
hgrepos = []
issue_num = 39225
keywords = []
message_count = 3.0
messages = ['359384', '359389', '359445']
nosy_count = 3.0
nosy_names = ['brett.cannon', 'eric.smith', 'reuven']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39225'
versions = []

[reuven]

Newcomers to Python are often frustrated and surprised when they define variables such as "sum" and "list", only to discover that they've masked access builtins of the same name.

External code checkers do help, but those don't work in Jupyter or other non-IDE environments.

It would be nice if defining a global/local with the same name as a builtin would generate a warning. For example:

list = [10, 20, 30]
RedefinedBuiltinWarning: "list" is a builtin, and should normally not be redefined. 

I'm sure that the wording could use a lot of work, but something like this would do wonders to help newbies, who encounter this all the time. Experienced developers are surprised that these terms aren't reserved words.

[ericvsmith]

On the other hand, this would generate a lot of warnings for users of libraries, who aren't able to do anything about it. I've got tons of existing code that uses the names of builtins as locals, to no harm. And as builtins are added, new warnings would pop up.

A linter is really the best way to handle this. Maybe Jupyter could add its own warning.

[brettcannon]

I agree with Eric that this is the realm of linters as there are legitimate reasons sometimes to mask and redefine built-ins.

Thanks for the idea, Reuven, but I'm closing the issue.