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copied!<p>The ASCII encoding only includes the bytes with values <code><= 127</code>. The range of characters represented by these bytes is identical in most encodings; in other words, "A" is <code>chr(65)</code> in ASCII, in latin-1, in UTF-8, and so on.</p> <p>The one half symbol, however, is not part of the ASCII character set, so when Python tries to encode this symbol into ASCII, it can do nothing but fail.</p> <p><strong>Update:</strong> Here's what happens (I assume we're talking CPython):</p> <p><code>float(u'\xbd')</code> leads to <code>PyFloat_FromString</code> in <a href="http://svn.python.org/view/python/branches/release26-maint/Objects/floatobject.c?revision=72566&view=markup" rel="nofollow noreferrer">floatobject.c</a> being called. This function, giving a unicode object, in turn calls <code>PyUnicode_EncodeDecimal</code> in <a href="http://svn.python.org/view/python/branches/release26-maint/Objects/unicodeobject.c?revision=74576&view=markup" rel="nofollow noreferrer">unicodeobject.c</a> being called. From skimming over the code, I get it that this function turns the unicode object into a string by replacing every character with a unicode codepoint <code><256</code> with the byte of that value, i.e. the one half character, having the codepoint 189, is turned into <code>chr(89)</code>.</p> <p>Then, <code>PyFloat_FromString</code> does its work as usual. At this moment, it's working with a regular string, which happens to be containing a non-ASCII range byte. It doesn't care about this; it just finds a byte that's not a digit, a period or the like, so it raises the value error.</p> <p>The argument to this exception is a string </p> <pre><code>"invalid literal for float(): " + evil_string </code></pre> <p>That's fine; an exception message is, after all, a string. It's only when you try to decode this string, using the default encoding ASCII, that this turns into a problem.</p>

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