New submission from Greg Ward:
Python's error message when you let None accidentally sneak into an expression
where it doesn't belong could be better. The canonical example is attribute
lookup:
>>> a = None
>>> a.foo
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'foo'
This assumes that the programmer knows there is only one object of type
NoneType, and it is None. That's a lot to assume of a beginner, whether they
are coming from another programming language ("null has a type? that's crazy
talk!") or are completely new to programming ("null? none? nil? wat...??").
There are plenty of other places this use of NoneType in error messages comes
up:
>>> a + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
>>> 1 + a
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'
>>> len(a)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'NoneType' has no len()
>>> a < 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'NoneType' and 'int'
I think we can do better than this. For example, here is an proposed
improvement to user experience for getting and setting attributes on None:
>>> a.foo
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: attempt to access attribute 'foo' of None, but None has no
attributes
>>> a.foo = 42
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: attempt to set attribute 'foo' on None, but None is read-only
Let the bikeshedding commence. I have a working patch, but need to write tests.
Will upload it here when that is done.
----------
assignee: gward
components: Interpreter Core
messages: 280884
nosy: gward
priority: normal
severity: normal
status: open
title: Confusing error message when None used in expressions, eg. "'NoneType'
object has no attribute 'foo'"
type: enhancement
versions: Python 3.6
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Python tracker <[email protected]>
<http://bugs.python.org/issue28702>
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