<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html><head><meta content="text/html; charset=ISO-8859-1" http-equiv="content-type"><title>Limitations</title></head> <body><h1><hr width="100%"> <a name="Limitations"></a>Limitations <hr width="100%"></h1><h2> <a name="Unsupported"></a>Unsupported Python features</h2> Pyrex is not quite a full superset of Python. The following restrictions apply: <blockquote> <li> Function definitions (whether using <b>def</b> or <b>cdef</b>) cannot be nested within other function definitions.<br> </li> <li> Class definitions can only appear at the top level of a module, not inside a function.<br> </li> <li> The<tt> import *</tt> form of import is not allowed anywhere (other forms of the import statement are fine, though).<br> </li> <li> Generators cannot be defined in Pyrex.<br> <br> </li> <li> The <tt>globals()</tt> and <tt>locals()</tt> functions cannot be used.</li> </blockquote> The above restrictions will most likely remain, since removing them would be difficult and they're not really needed for Pyrex's intended applications. <p>There are also some temporary limitations, which may eventually be lifted, including: </p> <blockquote> <li> Class and function definitions cannot be placed inside control structures.<br> </li> <li> List comprehensions are not yet supported.<br> </li> <li> There is no support for Unicode.<br> </li> <li> Special methods of extension types cannot have functioning docstrings.<br> <br> </li> <li> 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.</li></blockquote><hr style="width: 100%; height: 2px;"><h2><a name="SemanticDifferences"></a>Semantic differences between Python and Pyrex</h2> <h3> Behaviour of class scopes</h3> 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 method<sup><font size="-2"><a href="#Footnote1">1</a></font></sup>. A consequence of this is that the usual idiom for using the classmethod and staticmethod functions, e.g. <blockquote> <pre>class Spam:</pre> <pre> def method(cls):<br> ...</pre><pre> method = classmethod(method)</pre> </blockquote> will not work in Pyrex. This can be worked around by defining the function <i>outside</i> the class, and then assigning the result of classmethod or staticmethod inside the class, i.e. <blockquote> <pre>def Spam_method(cls):<br> ...</pre> <pre>class Spam:</pre><pre> method = classmethod(Spam_method)</pre> </blockquote> <hr width="100%"><span style="font-weight: bold;">Footnotes</span><br><hr style="width: 100%; height: 2px;"><a name="Footnote1"></a>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.<br><br>--- </body></html>
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