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mypy should support optional tagged unions #8925

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chdsbd opened this issue May 30, 2020 · 1 comment · Fixed by #11207
Closed

mypy should support optional tagged unions #8925

chdsbd opened this issue May 30, 2020 · 1 comment · Fixed by #11207
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bug mypy got something wrong false-positive mypy gave an error on correct code priority-1-normal topic-strict-optional topic-typed-dict

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@chdsbd
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chdsbd commented May 30, 2020

Report Type: Bug

Example: https://mypy-play.net/?mypy=latest&python=3.8&gist=529e99849ebe65749d8568b743f21050

from typing import Literal, TypedDict, Union

class NewJob(TypedDict):
    event_type: Literal["new-job"]
    job_description: str

class CancelJob(TypedDict):
    event_type: Literal["cancel-job"]
    job_id: int

request: Union[NewJob, CancelJob, None]

# works
if request is not None:
    if request["event_type"] == "new-job":
        print("Starting new job", request["job_description"])

# fails with `main.py:20: error: TypedDict "CancelJob" has no key 'job_description'`
if request is not None and request["event_type"] == "new-job":
    print("Starting new job", request["job_description"])
  • What is the actual behavior/output?
    Mypy does not discriminate TypedDicts when None is part of the Union.
  • What is the behavior/output you expect?
    Mypy narrows a Union of TypedDict and None like it does for unions of TypedDicts.
  • What are the versions of mypy and Python you are using?
    mypy 0.770
    Python 3.7.6
  • Do you see the same issue after installing mypy from Git master?
    I have not tried master.

This is a small issue because there is an easy workaround of checking for None before checking the tag.
@JukkaL JukkaL added bug mypy got something wrong false-positive mypy gave an error on correct code priority-1-normal topic-strict-optional topic-typed-dict labels Jun 8, 2020
@JukkaL
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JukkaL commented Jun 8, 2020

This looks like a bug.

@euri10 euri10 mentioned this issue Aug 14, 2020
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bluetech added a commit to bluetech/mypy that referenced this issue Sep 27, 2021
@bluetech
Fix narrowing information not propagated in assignment and boolean ex…
12166a7 
…pressions

Fixes python#8925.

Consider the following narrowing (more realistically with TypedDicts),
and the current outcome:

```py
class A:
    tag: Literal["A"]

class B:
    tag: Literal["B"]

abo: A | B | None
if abo is not None and abo.tag == "A":
    reveal_type(abo.tag)  # Type is Literal["A"]
    reveal_type(abo)  # Type is A | B
```

The RHS of the comparison correctly takes into account the LHS, and the
`abo.tag` expression is correctly narrowed based on it, but this does
not propagate upward to then narrow `abo` to `A`.

The problem is that `and`/`or/`not`/assignment expressions recurse using
the `find_isinstance_check_helper` function, which omits the
`propagate_up_typemap_info` calls that its parent function
`find_isinstance_check` performs.

Fix this by replacing these recursive `find_isinstance_check_helper`
calls with `find_isinstance_check`. This might not always be necessary,
but I do not think that always propagating upwards should do any harm,
anyways.
@bluetech bluetech mentioned this issue Sep 27, 2021
Merged
bluetech added a commit to bluetech/mypy that referenced this issue Sep 27, 2021
@bluetech
Fix narrowing information not propagated in assignment and boolean ex…
f3a37f7 
…pressions

Fixes python#8925.

Consider the following narrowing (more realistically with TypedDicts),
and the current outcome:

```py
class A:
    tag: Literal["A"]

class B:
    tag: Literal["B"]

abo: A | B | None
if abo is not None and abo.tag == "A":
    reveal_type(abo.tag)  # Type is Literal["A"]
    reveal_type(abo)  # Type is A | B
```

The RHS of the comparison correctly takes into account the LHS, and the
`abo.tag` expression is correctly narrowed based on it, but this does
not propagate upward to then narrow `abo` to `A`.

The problem is that `and`/`or/`not`/assignment expressions recurse using
the `find_isinstance_check_helper` function, which omits the
`propagate_up_typemap_info` calls that its parent function
`find_isinstance_check` performs.

Fix this by replacing these recursive `find_isinstance_check_helper`
calls with `find_isinstance_check`. This might not always be necessary,
but I do not think that always propagating upwards should do any harm,
anyways.
JukkaL pushed a commit that referenced this issue Oct 1, 2021
@bluetech
Fix narrowing information not propagated in assignment and boolean ex…
02b9694 
…pressions (#11207)

Fixes #8925.

Consider the following narrowing (more realistically with TypedDicts),
and the current outcome:

```py
class A:
    tag: Literal["A"]

class B:
    tag: Literal["B"]

abo: A | B | None
if abo is not None and abo.tag == "A":
    reveal_type(abo.tag)  # Type is Literal["A"]
    reveal_type(abo)  # Type is A | B
```

The RHS of the comparison correctly takes into account the LHS, and the
`abo.tag` expression is correctly narrowed based on it, but this does
not propagate upward to then narrow `abo` to `A`.

The problem is that `and`/`or/`not`/assignment expressions recurse using
the `find_isinstance_check_helper` function, which omits the
`propagate_up_typemap_info` calls that its parent function
`find_isinstance_check` performs.

Fix this by replacing these recursive `find_isinstance_check_helper`
calls with `find_isinstance_check`. This might not always be necessary,
but I do not think that always propagating upwards should do any harm,
anyways.
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Fix narrowing information not propagated in assignment and boolean expressions bluetech/mypy
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@JukkaL @chdsbd