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Fixes #8481
Fixes #9941
Fixes #11025
Fixes #11038

This is the unresolved crash that's been reported the most times, e.g., it's as easy 
to repro as `mypy --no-implicit-reexport -c 'import pytorch_lightning'` (or even 
`import torch`, with the right PyTorch version). I know of multiple popular Python 
packages that have made changes just to work around this crash. I would love to 
see this released, potentially along with #11630.

Thanks to @rraval for making a minimal repro!

The fix ended up being a one-liner, but it took me a bit to track down :-)

Since things worked with implicit reexport, I differentially patched in explicit reexport 
logic to narrow things down. This let me observe that if I hardcoded pkg.a.B to get 
reexported, we hit this branch, which clobbers the PlaceholderNode for pkg.c.B, 
which fixes things:

Which is a little weird — we shouldn't have a PlaceholderNode for pkg.c.B at all. But 
with a breakpoint in that branch, it was easy to notice that with `--no-implicit-reexport` 
pkg.a.B was first created with `module_public=True` (resulting in creation of a 
PlaceholderNode in pkg.c.B) and only on a later pass acquired `module_public=False`. 
So tracking down where pkg.a.B symbol was first created with `module_public=True` 
led me to this "obvious" bug.

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Mypy: Static Typing for Python

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Got a question?

We are always happy to answer questions! Here are some good places to ask them:

If you're just getting started, the documentation and type hints cheat sheet can also help answer questions.

If you think you've found a bug:

To report a bug or request an enhancement:

To discuss a new type system feature:

What is mypy?

Mypy is a static type checker for Python.

Type checkers help ensure that you're using variables and functions in your code correctly. With mypy, add type hints (PEP 484) to your Python programs, and mypy will warn you when you use those types incorrectly.

Python is a dynamic language, so usually you'll only see errors in your code when you attempt to run it. Mypy is a static checker, so it finds bugs in your programs without even running them!

Mypy is designed with gradual typing in mind. This means you can add type hints to your code base slowly and that you can always fall back to dynamic typing when static typing is not convenient.

Here is a small example to whet your appetite:

number = input("What is your favourite number?")
print("Well, my favourite number is: ", number + 1)  # error: Unsupported operand types for + ("str" and "int")

See the documentation for more examples.

In particular, see:

Quick start

Mypy can be installed using pip:

python3 -m pip install -U mypy

If you want to run the latest version of the code, you can install from git:

python3 -m pip install -U git+git://

Now you can type-check the statically typed parts of a program like this:


You can always use the Python interpreter to run your statically typed programs, even if mypy reports type errors:

python3 PROGRAM

You can also try mypy in an online playground (developed by Yusuke Miyazaki).


Mypy can be integrated into popular IDEs:

Web site and documentation

Additional information is available at the web site:

Jump straight to the documentation:

Follow along our changelog at:


Help in testing, development, documentation and other tasks is highly appreciated and useful to the project. There are tasks for contributors of all experience levels.

To get started with developing mypy, see

If you need help getting started, don't hesitate to ask on gitter.

Development status

Mypy is beta software, but it has already been used in production for several years at Dropbox and in many other organizations, and it has an extensive test suite.

mypyc and compiled version of mypy

Mypyc uses Python type hints to compile Python modules to faster C extensions. Mypy is itself compiled using mypyc: this makes mypy approximately 4 times faster than if interpreted!

To install an interpreted mypy instead, use:

python3 -m pip install --no-binary mypy -U mypy

To use a compiled version of a development version of mypy, directly install a binary from

To contribute to the mypyc project, check out