`ElementTree.parse` type hinted to return unusable generic types
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Description
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 thanET.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 ofET.parse(), it returns the root-node as a type that can be assigned toET.Elementwithout 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 suggestingET.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:
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 by reading typeshed PR 13349 and the annotations for ElementTree.parse, then compare them with the ElementTree().parse() workaround and the Python ElementTree documentation. Done means the intended generic behavior and the expected assignability of ET.parse() are established, with the relevant typing or documentation issue addressed.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100