python / python/mypy

Inconsistent "cannot instantiate abstract class" with list of concete types deriving from an abstract base

Open
#15,554 1 comment 3 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

bug topic-join-v-union
Dominant language
Python
Stars
20.6k
Forks
3.3k
PR merge metrics
PR metrics pending

Description

Bug Report

mypy gives inconsistent errors with regard to instantiating classes that derive from a common abstract base class when the derived classes are accessed via a list.

The errors depend on size of the list, and whether the type of the list is inferred or explicit.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=0fb05ed25d3fd593772c6a7586f92929

Expected Behavior

Each example with the same inferred/explicit typing should give the same errors.

Actual Behavior

The example with a list of three elements and inferred type gives an error where all other examples don't

Your Environment

  • Mypy version used: 1.4.1
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.10

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the linked mypy-play reproduction using mypy 1.4.1 and compare the inferred and explicitly typed lists, especially the three-element case. Trace the abstract-class instantiation checking involved in those examples; done means equivalent inferred and explicit typings produce consistent errors.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
tooling
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.