Inconsistent "cannot instantiate abstract class" with list of concete types deriving from an abstract base
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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
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- 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