8000 Gradually building typed dict · Issue #2494 · python/mypy · GitHub
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JukkaL opened this issue Nov 24, 2016 · 1 comment
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Gradually building typed dict #2494

JukkaL opened this issue Nov 24, 2016 · 1 comment
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needs discussion priority-1-normal topic-typed-dict

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@JukkaL
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JukkaL commented Nov 24, 2016

We may want to support code like this, since this style of creating dictionaries seems pretty common:

X = TypedDict('X', {'n': int, 'status': str})

def f() -> X
    x = {}  # type: X
    x['n'] = 3
    if cond():
        x['status'] = 'yes'
    else:
        x['status'] = 'no'
    return x

We could perhaps use partial types and the conditional type binder for this. The type of x would be partial until all items have been set.

This code would be rejected, since x is still partial when we read it:

def f() -> X:
    x = {}  # type: X
    x['n'] = 3
    if cond():
        x['status'] = 'yes'
    return x  # error: "status" of "x" may be undefined
@JukkaL JukkaL mentioned this issue Jun 28, 2018
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@JukkaL
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JukkaL commented Jan 28, 2020

This seems too complicated to implement relative to the potential benefit.

@JukkaL JukkaL closed this as completed Jan 28, 2020
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