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Statically type a subset of Python 3

Project Description

Nope is a statically-typed subset of Python 3 that can be compiled to multiple targets. At the moment, only Python and node.js are supported. Any valid Nope program can be run directly as a Python 3 program.

Static typing is supported for two main reasons:

  • Static typing can detect some programming errors more quickly
  • Static typing allows optimisations to be applied to the generated code

The static types are expressed using comments, rather than annotations, for several reasons:

  • This means the static typing has no effect at runtime, allowing Nope programs to be run directly as Python 3 programs without any extra dependencies or performance penalty.
  • A separate syntax can be used within the comments to succinctly express types, rather than (ab)using existing Python syntax.
  • It can be useful to decouple the signature of a function from the implementation. For instance, using separate comments makes it easy to type a function such that it only accepts arguments by positions rather than keyword.

Here’s an example of calculating Fibonacci numbers using Nope:

#:: int -> int
def fib(n):
    seq = [0, 1]
    for i in range(2, n + 1):
        seq.append(seq[i - 1] + seq[i - 2])

    return seq[n]



  • When defining __add__ and similar methods on classes, the type signature should be specific e.g. on int, int -> int. However, to maintain compatibility with Python, the type checker should assume the argument is the top type when type checking the actual method, so isinstance or similar still has to be used.
  • Support for the r versions of operators e.g. __radd__.
  • Inheritance
  • __init__ methods
  • Standard library support
  • A way of specifying dependencies on a per-platform basis to allow shimming of existing libraries into a common interface.
  • Allow types of variables to be specified in the same way as functions, such as giving the type of an empty list.
  • If a class definition body contains a value of type object that could be a function (but that is not possible to determine at runtime), how should it be treated? In Python, if it’s a function, we bind it to the instance. Is it possible to sensibly do the same in other languages? The result is that any value of type object will need to be checked as to whether it is a function or not for consistency.
  • Proper tests for builtin functions
  • Prevent re-definition of functions and classes
  • Allow mutually recursive functions
  • Ensure that all signatures are used in typing rules



This section describes support for parsing and type-checking each of Python 3.4’s syntax nodes. Note that not all backends may support all features.

  • Function definitions: partially supported.

    • name: supported.
    • arguments: partially supported. Positional and keyword arguments are supported, but nothing else (default values, *args, **kwargs, keyword-only arguments).
    • body: supported.
    • decorators: unsupported.
    • annotations: unsupported (both argument and return annotations).

    The signature of a function should be specified by a signature comment immediately before the function definition. For instance:

    #:: int -> int
    def increment(x):
        return x + 1
    #:: int, str -> none
    def repeated_greeting(repeat, message):
        for i in range(0, repeat):
    #:: repeat: int, message: str -> none
    def repeated_greeting(repeat, message):
        for i in range(0, repeat):
  • Class definitions: unsupported.

  • Return statements: supported.

  • Delete statements: unsupported.

  • Assignments: partially supported. Assignments to variables (e.g. x), elements of sequences (e.g. x[i]), and attributes (e.g. x.y) are supported, but not assignment to slices (e.g. x[:]).

  • Augmented assignments: unsupported.

  • For loops: supported.

  • While loops: supported.

  • If statements: supported.

  • With statements: supported.

  • Raise statements: partially supported. Only statements in the form raise value are supported. raise, raise ExceptionType and raise value1 from value2 are unsupported.

  • Try statements: partially supported. Tuples of exceptions are not supported when specifying the type in exception handlers. The else branch is ignored.

  • Assert statements: supported.

  • Import statements: partially supported. The various forms of import statement are supported. However, only local modules are currently supported. Modules from the standard library or dependencies are unsupported.

  • global keyword: unsupported.

  • nonlocal keyword: unsupported.

  • Expression statements: supported.

  • pass keyword: supported.

  • break keyword: supported.

  • continue keyword: supported.

With statements

Consider the following:

with x:
    y = f()


It isn’t guaranteed that y has been assigned a value since f() could raise an exception that is then suppressed by the context manager’s __exit__ method. Therefore, g(y) fails to type-check. (If the exception isn’t suppressed by the __exit__ method, we can safely assume treat the variable as assigned since we won’t be executing any code after the exception). However, in the common case, we’d like to be able to assume that the variable has been assigned, and such an assumption is safe in many cases, such as:

with open(path) as file_:
    contents =


We can allow such examples to type-check by inspecting the type of __exit__. If its return type is none, then it is guaranteed to return a false value, meaning it will never suppress exceptions.


Any valid Nope program should be directly executable using Python 3.4. The best way to support earlier versions of Python is in the same way as you would on a normal Python 3.4 codebase i.e. avoiding features unsupported in earlier versions.

Node.js backend

Supported builtin functions:

  • abs: supported
  • bool: partially supported. The magic method __bool__ is ignored.
  • iter: partially supported. The sequence protocol is unsupported.
  • print: only a single argument is accepted.

Unimplemented optimisations:

  • If the result of boolean operations (‘and’ or ‘or’) is only used as a condition, such as the condition of an ‘if’ statement or ‘while’ loop, then the value can simply be true or false rather than the actual value of the operation. In other words, x and y can be optimised to bool(x) && bool(y).
  • Unless bool() has been explicitly invoked, booleans, strings and integers can be used directly if only used for their truth value e.g. in if statement conditions.
  • Avoid re-evaluating bool(value) if boolean operations are used directly in conditions. For instance, in if x and y, bool(x) only needs to be evaluated once, even if bool(x) is True. (A naive implementation evalutes bool(x) once for the and operation, which would have the value of x, causing bool(x) to be evaluated again as the condition of the if statement.)

Differences from Python 3

Subclassing builtins

Nope does not allow subclassing of some builtins, such as int and list. This restraint means a value of type int is guaranteed to have the concrete type int rather than a subclass of int, allowing certain optimisations to be used when generating code.

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