Limitations

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Limitations

## Unsupported Python features

Pyrex is not quite a full superset of Python. The following restrictions apply:

> - Function definitions (whether using def or cdef) cannot be nested within other function definitions.
> - Class definitions can only appear at the top level of a module, not inside a function.
> - The`import *` form of import is not allowed anywhere (other forms of the import statement are fine, though).
> - Generators cannot be defined in Pyrex.
> - The `globals()` and `locals()` functions cannot be used.

The above restrictions will most likely remain, since removing them would be difficult and they're not really needed for Pyrex's intended applications.

There are also some temporary limitations, which may eventually be lifted, including:

> - Class and function definitions cannot be placed inside control structures.
> - List comprehensions are not yet supported.
> - There is no support for Unicode.
> - Special methods of extension types cannot have functioning docstrings.
> - The use of string literals as comments is not recommended at present, because they are not accepted in places where executable statements are not allowed.

## Semantic differences between Python and Pyrex

### Behaviour of class scopes

In Python, referring to a method of a class inside the class definition, i.e. while the class is being defined, yields a plain function object, but in Pyrex it yields an unbound method1. A consequence of this is that the usual idiom for using the classmethod and staticmethod functions, e.g.

>     class Spam:

>       def method(cls):
>         ...

>       method = classmethod(method)

will not work in Pyrex. This can be worked around by defining the function outside the class, and then assigning the result of classmethod or staticmethod inside the class, i.e.

>     def Spam_method(cls):
>       ...

>     class Spam:

>       method = classmethod(Spam_method)

Footnotes

1. The reason for the different behaviour of class scopes is that Pyrex-defined Python functions are PyCFunction objects, not PyFunction objects, and are not recognised by the machinery that creates a bound or unbound method when a function is extracted from a class. To get around this, Pyrex wraps each method in an unbound method object itself before storing it in the class's dictionary.
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