Differences from Python

Mypyc aims to be sufficiently compatible with Python semantics so that migrating code to mypyc often doesn’t require major code changes. There are various differences to enable performance gains that you need to be aware of, however.

This section documents notable differences from Python. We discuss many of them also elsewhere, but it’s convenient to have them here in one place.

Running compiled modules

You can’t use python3 <module>.py or python3 -m <module> to run compiled modules. Use python3 -c "import <module>" instead, or write a wrapper script that imports your module.

As a side effect, you can’t rely on checking the __name__ attribute in compiled code, like this:

if __name__ == "__main__":  # Can't be used in compiled code

Type errors prevent compilation

You can’t compile code that generates mypy type check errors. You can sometimes ignore these with a # type: ignore comment, but this can result in bad code being generated, and it’s considered dangerous.


In the future, mypyc may reject # type: ignore comments that may be unsafe.

Runtime type checking

Non-erased types in annotations will be type checked at runtime. For example, consider this function:

def twice(x: int) -> int:
    return x * 2

If you try to call this function with a float or str argument, you’ll get a type error on the call site, even if the call site is not being type checked:

twice(5)  # OK
twice(2.2)  # TypeError
twice("blah")  # TypeError

Also, values with inferred types will be type checked. For example, consider a call to the stdlib function socket.gethostname() in compiled code. This function is not compiled (no stdlib modules are compiled with mypyc), but mypyc uses a library stub file to infer the return type as str. Compiled code calling gethostname() will fail with TypeError if gethostname() would return an incompatible value, such as None:

import socket

# Fail if returned value is not a str
name = socket.gethostname()

Note that gethostname() is defined like this in the stub file for socket (in typeshed):

def gethostname() -> str: ...

Thus mypyc verifies that library stub files and annotations in non-compiled code match runtime values. This adds an extra layer of type safety.

Casts such as cast(str, x) will also result in strict type checks. Consider this example:

from typing import cast
x = cast(str, y)

The last line is essentially equivalent to this Python code when compiled:

if not isinstance(y, str):
    raise TypeError(...)
x = y

In interpreted mode cast does not perform a runtime type check.

Native classes

Native classes behave differently from Python classes. See Native classes for the details.

Primitive types

Some primitive types behave differently in compiled code to improve performance.

int objects use an unboxed (non-heap-allocated) representation for small integer values. A side effect of this is that the exact runtime type of int values is lost. For example, consider this simple function:

def first_int(x: List[int]) -> int:
    return x[0]

print(first_int([True]))  # Output is 1, instead of True!

bool is a subclass of int, so the above code is valid. However, when the list value is converted to int, True is converted to the corresponding int value, which is 1.

Note that integers still have an arbitrary precision in compiled code, similar to normal Python integers.

Fixed-length tuples are unboxed, similar to integers. The exact type and identity of fixed-length tuples is not preserved, and you can’t reliably use is checks to compare tuples that are used in compiled code.

Early binding

References to functions, types, most attributes, and methods in the same compilation unit use early binding: the target of the reference is decided at compile time, whenever possible. This contrasts with normal Python behavior of late binding, where the target is found by a namespace lookup at runtime. Omitting these namespace lookups improves performance, but some Python idioms don’t work without changes.

Note that non-final module-level variables still use late binding. You may want to avoid these in very performance-critical code.

Examples of early and late binding:

from typing import Final

import lib  # "lib" is not compiled

x = 0
y: Final = 1

def func() -> None:

class Cls:
    def __init__(self, attr: int) -> None:
        self.attr = attr

    def method(self) -> None:

def example() -> None:
    # Early binding:
    var = y
    o = Cls()

    # Late binding:
    var = x  # Module-level variable
    lib.func()  # Accessing library that is not compiled

Pickling and copying objects

Mypyc tries to enforce that instances native classes are properly initialized by calling __init__ implicitly when constructing objects, even if objects are constructed through pickle, copy.copy or copy.deepcopy, for example.

If a native class doesn’t support calling __init__ without arguments, you can’t pickle or copy instances of the class. Use the mypy_extensions.mypyc_attr class decorator to override this behavior and enable pickling through the serializable flag:

from mypy_extensions import mypyc_attr
import pickle

class Cls:
    def __init__(self, n: int) -> None:
        self.n = n

data = pickle.dumps(Cls(5))
obj = pickle.loads(data)  # OK

Additional notes:

  • All subclasses inherit the serializable flag.

  • If a class has the allow_interpreted_subclasses attribute, it implicitly supports serialization.

  • Enabling serialization may slow down attribute access, since compiled code has to be always prepared to raise AttributeError in case an attribute is not defined at runtime.

  • If you try to pickle an object without setting the serializable flag, you’ll get a TypeError about missing arguments to __init__.

Monkey patching

Since mypyc function and class definitions are immutable, you can’t perform arbitrary monkey patching, such as replacing functions or methods with mocks in tests.


Each compiled module has a Python namespace that is initialized to point to compiled functions and type objects. This namespace is a regular dict object, and it can be modified. However, compiled code generally doesn’t use this namespace, so any changes will only be visible to non-compiled code.

Stack overflows

Compiled code currently doesn’t check for stack overflows. Your program may crash in an unrecoverable fashion if you have too many nested function calls, typically due to out-of-control recursion.


This limitation will be fixed in the future.

Final values

Compiled code replaces a reference to an attribute declared Final with the value of the attribute computed at compile time. This is an example of early binding. Example:

MAX: Final = 100

def limit_to_max(x: int) -> int:
     if x > MAX:
         return MAX
     return x

The two references to MAX don’t involve any module namespace lookups, and are equivalent to this code:

def limit_to_max(x: int) -> int:
     if x > 100:
         return 100
     return x

When run as interpreted, the first example will execute slower due to the extra namespace lookups. In interpreted code final attributes can also be modified.

Unsupported features

Some Python features are not supported by mypyc (yet). They can’t be used in compiled code, or there are some limitations. You can partially work around some of these limitations by running your code in interpreted mode.

Nested classes

Nested classes are not supported.

Conditional functions or classes

Function and class definitions guarded by an if-statement are not supported.

Dunder methods

Native classes cannot use these dunders. If defined, they will not work as expected.

  • __del__

  • __index__

  • __getattr__, __getattribute__

  • __setattr__

  • __delattr__

Generator expressions

Generator expressions are not supported. To make it easier to compile existing code, they are implicitly replaced with list comprehensions. This does not always produce the same behavior.

To work around this limitation, you can usually use a generator function instead. You can sometimes replace the generator expression with an explicit list comprehension.


Native classes can’t contain arbitrary descriptors. Properties, static methods and class methods are supported.


Various methods of introspection may break by using mypyc. Here’s an non-exhaustive list of what won’t work:

  • Instance __annotations__ is usually not kept

  • Frames of compiled functions can’t be inspected using inspect

  • Compiled methods aren’t considered methods by inspect.ismethod

  • inspect.signature chokes on compiled functions

Profiling hooks and tracing

Compiled functions don’t trigger profiling and tracing hooks, such as when using the profile, cProfile, or trace modules.


You can’t set breakpoints in compiled functions or step through compiled functions using pdb. Often you can debug your code in interpreted mode instead.