python / python/typeshed

`ElementTree.parse` type hinted to return unusable generic types

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Descripción

Bug Report

Since upgrading to mypy 1.15+, using ElementTree has become increasingly difficult. Likely due to https://github.com/python/typeshed/pull/13349. I'm not sure if this extra strictness is expected or if I'm just holding it wrong.

One example is the ElementTree.parse function that now returns a generic ET.ElementTree[ET.Element[str]], which is no longer compatible with basic type signatures like ET.ElementTree, which is interpreted as ElementTree[Element[str] | None].

This adds many challenges that make it difficult to work with.

  • The generic argument here is very difficult to understand what it means and is lacking documentation. From what i've been able to deduce it's possible that the root node is of a different Element type or that the root node is None. While perhaps a valid scenario, it adds a big uphill to learning and usability.
  • If one were to use these generic types, typing out ET.ElementTree[ET.Element[str]] is a lot more verbose than ET.ElementTree, and i'm questioning if the strictness here is worth it.
  • If one tries to type hint the code with these generics. Python throws errors that 'ElementTree.Element' is not subscriptable. One workaround i found for this is to surround the type hints with double-quotes.

To Reproduce

import xml.etree.ElementTree as ET

def process_document(doc: ET.ElementTree) -> None:
    pass

def process_document2(doc: ET.ElementTree[ET.Element[str]]) -> None:    # TypeError: type 'xml.etree.ElementTree.Element' is not subscriptable
    pass

process_document(ET.parse("foo.xml"))     # mypy error: Argument 1 to "process_document" has incompatible type "ElementTree[Element[str]]"; expected "ElementTree[Element[str] | None]"  [arg-type]
process_document2(ET.parse("foo.xml"))

Run with mypy --strict

Expected Behavior

ET.parse returns types assignable to ET.ElementTree

Actual Behavior

Mypy returns errror

# mypy error: Argument 1 to "process_document" has incompatible type "ElementTree[Element[str]]"; expected "ElementTree[Element[str] | None]"  [arg-type]

Workarounds

  • Use ET.ElementTree().parse() function instead of ET.parse(), it returns the root-node as a type that can be assigned to ET.Element without any generic arguments.
    This is actually a pretty decent option, if this is considered to be the primary way of using ElementTree from now on, the documentation should reflect that, right now it's suggesting ET.parse()
  • Type out the complete "ElementTree[Element[str] | None]" wrapped with double-quotes.

Your Environment

  • Mypy version used:
  • Mypy command-line flags:
  • Mypy configuration options from mypy.ini (and other config files):
  • Python version used:

Guía de contribución

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Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

Línea de trabajo

Empieza leyendo typeshed PR 13349 y las anotaciones de ElementTree.parse; después, compáralas con la solución alternativa ElementTree().parse() y la documentación de Python ElementTree. Se considera terminado cuando se hayan establecido el comportamiento genérico previsto y la asignabilidad esperada de ET.parse(), y se haya abordado el problema de tipado o documentación correspondiente.

Escrito por el modelo de indexación a partir del texto del issue.

Evaluación

Stack tecnológico
python
Área
devtools
Tipo de issue
Error
Dificultad
4/5
Tiempo estimado
3-5 días
Estado de actividad
Estancado
Claridad
Bastante claro
Aptitud para principiantes
35/100

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