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53 lines
3.2 KiB
53 lines
3.2 KiB
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
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<html><head><meta content="text/html; charset=ISO-8859-1" http-equiv="content-type"><title>Limitations</title></head>
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<body><h1><hr width="100%">
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<a name="Limitations"></a>Limitations <hr width="100%"></h1><h2> <a name="Unsupported"></a>Unsupported
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Python features</h2>
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Pyrex is not quite a full superset of Python. The following
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restrictions apply: <blockquote> <li> Function
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definitions (whether using <b>def</b> or <b>cdef</b>)
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cannot be nested within other function definitions.<br> </li>
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<li> Class definitions can only appear at the top
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level of a module, not inside a function.<br> </li>
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<li> The<tt> import *</tt> form of import
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is not allowed anywhere (other forms of the import statement are fine,
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though).<br> </li> <li> Generators
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cannot be defined in Pyrex.<br> <br> </li> <li>
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The <tt>globals()</tt> and <tt>locals()</tt>
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functions cannot be used.</li> </blockquote> The above
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restrictions will most likely remain, since removing them would be
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difficult and they're not really needed for Pyrex's intended
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applications. <p>There are also some temporary limitations,
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which may eventually be lifted, including: </p> <blockquote>
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<li> Class and function definitions cannot be placed inside
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control structures.<br> </li> <li> List comprehensions are not yet
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supported.<br> </li> <li> There is no
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support for Unicode.<br> </li> <li>
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Special methods of extension types cannot have functioning
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docstrings.<br> <br> </li> <li> The use of
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string literals as comments is not recommended at present, because they are not accepted in
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places where executable statements are not allowed.</li></blockquote><hr style="width: 100%; height: 2px;"><h2><a name="SemanticDifferences"></a>Semantic
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differences between Python and Pyrex</h2> <h3> Behaviour
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of class scopes</h3> In Python, referring to a method of a class
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inside the class definition, i.e. while the class is being defined,
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yields a plain function object, but in Pyrex it yields an unbound method<sup><font size="-2"><a href="#Footnote1">1</a></font></sup>.
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A consequence of this is that the
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usual idiom for using the classmethod and staticmethod functions, e.g. <blockquote>
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<pre>class Spam:</pre> <pre> def method(cls):<br> ...</pre><pre> method = classmethod(method)</pre>
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</blockquote>
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will not work in Pyrex. This can be worked around by defining the
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function <i>outside</i> the class, and then assigning the
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result of classmethod or staticmethod inside the class, i.e. <blockquote>
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<pre>def Spam_method(cls):<br> ...</pre> <pre>class Spam:</pre><pre> method = classmethod(Spam_method)</pre>
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</blockquote> <hr width="100%"><span style="font-weight: bold;">Footnotes</span><br><hr style="width: 100%; height: 2px;"><a name="Footnote1"></a>1.
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The reason for the different
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behaviour
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of class scopes is that Pyrex-defined Python functions are PyCFunction
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objects,
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not PyFunction objects, and are not recognised by the machinery that
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creates
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a bound or unbound method when a function is extracted from a class. To
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get
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around this, Pyrex wraps each method in an unbound method object itself
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before
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storing it in the class's dictionary.<br><br>--- </body></html>
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