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    <p>From CPython source, <code>Objects/frameobject.c</code>:</p> <pre class="lang-c prettyprint-override"><code>static PyMemberDef frame_memberlist[] = { {"f_back", T_OBJECT, OFF(f_back), RO}, {"f_code", T_OBJECT, OFF(f_code), RO}, {"f_builtins", T_OBJECT, OFF(f_builtins),RO}, {"f_globals", T_OBJECT, OFF(f_globals), RO}, {"f_lasti", T_INT, OFF(f_lasti), RO}, {"f_exc_type", T_OBJECT, OFF(f_exc_type)}, {"f_exc_value", T_OBJECT, OFF(f_exc_value)}, {"f_exc_traceback", T_OBJECT, OFF(f_exc_traceback)}, {NULL} /* Sentinel */ }; ... static PyGetSetDef frame_getsetlist[] = { {"f_locals", (getter)frame_getlocals, NULL, NULL}, {"f_lineno", (getter)frame_getlineno, (setter)frame_setlineno, NULL}, {"f_trace", (getter)frame_gettrace, (setter)frame_settrace, NULL}, {"f_restricted",(getter)frame_getrestricted,NULL, NULL}, {0} }; </code></pre> <p>For the <code>PyMemberDef</code>, the flags <code>RO</code> or <code>READONLY</code> means it's attributes are read-only. For the <code>PyGetSetDef</code>, if it only has a getter, it's read only. This means all attributes but <code>f_exc_type</code>, <code>f_exc_value</code>, <code>f_exc_traceback</code> and <code>f_trace</code> are read-only after creation. This is also mentioned in the docs, under <a href="http://docs.python.org/reference/datamodel.html#index-1767" rel="nofollow noreferrer">Data model</a>.</p> <p>The objects referred to by the attributes is not necessarily read-only. You could do this:</p> <pre><code>&gt;&gt;&gt; f = sys._getframe() &gt;&gt;&gt; f.f_locals['foo'] = 3 &gt;&gt;&gt; foo 3 &gt;&gt;&gt; </code></pre> <p>Though this works in the interpreter, it fails inside functions. The execution engine uses a separate array for local variables (<code>f_fastlocals</code>), which is merged into <code>f_locals</code> on access, but the converse is not true.</p> <pre><code>&gt;&gt;&gt; def foo(): ... x = 3 ... f = sys._getframe() ... print f.f_locals['x'] ... x = 4 ... print f.f_locals['x'] ... d = f.f_locals ... x = 5 ... print d['x'] ... f.f_locals ... print d['x'] ... &gt;&gt;&gt; foo() 3 4 4 5 &gt;&gt;&gt; </code></pre> <p>On the global frame, <code>f_local</code> refers to <code>f_globals</code>, which makes this trick work in the interpreter. Modifying <code>f_globals</code> works, but affects the whole module.</p>
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    1. POIs it possible to write to a python frame object as returned by sys._getframe() from python code running within the interpreter?
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    1. USMarkus Jarderot
    1. USMarkus Jarderot
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    1. POIs it possible to write to a python frame object as returned by sys._getframe() from python code running within the interpreter?
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    1. COInteresting. This seems to be the behaviour on an interactive shell, but f_locals seems to be read only on a script. See my expanded question - this code snippet run in an interpreter appears to be read only.
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      1. USConcernedOfTunbridgeWells
 

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