Improve "not found" error messages in `<sequence>.<method>()` failures

[hyperkai]

The docstring of list.remove() says value parameter as shown below:

print(help(list.remove))
# remove(self, value, /) unbound builtins.list method
#     Remove first occurrence of value.
#
#     Raises ValueError if the value is not present.

But the error message says x instead of value as shown below and I don't think list.remove(x): is necessary:

v = ['a', 'b', 'c']

v.remove('B')
# ValueError: list.remove(x): x not in list

So, the error message should just say as shown below:

ValueError: value not in list

In addition, the docstring of bytearray.remove() says valueparameter as shown below:

print(help(bytearray.remove))
# remove(self, value, /) unbound builtins.bytearray method
#     Remove the first occurrence of a value in the bytearray.
#
#     value
#       The value to remove.

And, the error message reasonably says value as shown below but I don't think found is necessary:

v = bytearray(b'abc')

v.remove(ord('B'))
# ValueError: value not found in bytearray

So, the error message should just say as shown below:

ValueError: value not in bytearray

Linked PRs

• gh-137508: Improve list.remove/index error message #137509
• gh-137508: Unify error messages for index() and remove() #139696

[Eclips4]

Hello!
Did you mean to change the error message for v.remove('B') from ValueError: list.remove(x): x not in list to ValueError: 'B' is not in list?

If so, this is a duplicate of #121288 and related issues.
I don't think we should change anything, the current approach seems fine to me.
cc @serhiy-storchaka

[serhiy-storchaka]

I do not think that the OP proposes to include the repr of the value in the error message. They only propose to use other name than "x". For example, "value", which is used in the documentation and the help output. There is also a difference in wording between list.remove() and bytearray.remove(), so we can use opportunity to unify error messages.

[serhiy-storchaka]

I am not qualified to judge which wording is better. We need the help of English experts.

• Should the method name (list.remove(), bytearray.remove(), etc) be included?
• Should the method name be used with argument or not? list.remove(value): ... vs list.remove(): ...
• Should "found" be used?
• Should "is" be used?
• Should an article be used?
• For subclasses, should the subclass name be used, or only the name of the base class?

Note that we have the same or similar error messages for index() methods. This includes also immutable classes.

>>> [].index(1)
ValueError: list.index(x): x not in list
>>> ''.index('x')
ValueError: substring not found
>>> bytearray().index(b'x')
ValueError: subsection not found
>>> bytearray().index(1)
ValueError: subsection not found

(BTW, I think that error messages should be different for searching an integer and a byte string in a bytearray).

[picnixz]

I'm recategorizing this as a feature but we really need more people for this discussion. Note that changing those messages also mean breaking doctests that rely on that and all other test suites. For classes that are not widely-used, I think it's fine. But for builtins we need to be careful.