[Feature Request] Provide an API to get the type of the expressions in the AST

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#4,868 25 comments 10 reactions 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
20/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
developer-experience, tooling

Research direction

Start by reviewing mypy's AST and type-inference entry points for expressions identified by source location; the issue names no files or tests to begin with. Compare the requested API with the separate heuristics used by jedi and pylint, then define an interface that can expose expression types and document tests showing that type queries support safe refactoring.

Written by the indexing model from the issue text.

Description

feature needs discussion priority-1-normal

Knowing the type of the expressions in the AST is useful for IDEs (and IDE plugins) to support autocompletion, for static analyzers and for refactoring tools. As a matter of fact, both jedi and pylint have their own heuristics for type inference.

In my specific use case I would like to use the type information to write a safe refactoring tool. The idea is to be able to say that you want to refactor specific methods. For example one could want to refactor string.find into string.index, as:

pos = expr.find(x)
if  pos >= 0:
  # do something with pos
else:
  # do something else

where expr is any string expression, like 'string', string_var, (var + 'foo'), string_var.replace(' ', '-'), etc.

into

try:
  pos = var.index(x)
  # do something with pos
except ValueError:
  # do something else

To safely do this refactoring, one needs to be able to query the type of sub expressions, given their location in the source file. Otherwise, given that find is a common name present in many different classes, the refactoring, would blindly be applied to all of them.

Note: this is the feature request version of issue #4713.

Dominant language
Python
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