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POWhat happens to the variable in this case when it is overwritten with self?
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copied!<p>I am downloading URLs in Python and need to detect 404s, so after some search I came up with:</p> <pre><code>import urllib class MyUrlOpener(urllib.FancyURLopener): def retrieve(self, url, filename=None, reporthook=None, data=None): self.file_was_found = True val = urllib.FancyURLopener.retrieve(self, url, filename, reporthook, data) return val def http_error_404(url, fp, errcode, errmsg, headers, data): url.file_was_found = False def download_file(url, saveas): urlaccess = MyUrlOpener() localFile, headers = urlaccess.retrieve(url, saveas) return urlaccess.file_was_found </code></pre> <p>My question is that if you look at the source code (Python 2.7) for FancyURLopener then you see: </p> <pre><code>def http_error(self, url, fp, errcode, errmsg, headers, data=None): """Handle http errors. Derived class can override this, or provide specific handlers named http_error_DDD where DDD is the 3-digit error code.""" # First check if there's a specific handler for this error name = 'http_error_%d' % errcode if hasattr(self, name): method = getattr(self, name) if data is None: result = method(url, fp, errcode, errmsg, headers) else: result = method(url, fp, errcode, errmsg, headers, data) if result: return result return self.http_error_default(url, fp, errcode, errmsg, headers) </code></pre> <p>Which is passing the <code>url</code> as the first parameter and not <code>self</code>. I thought that the first parameter to a function was always a reference to the class instance (by convention) and my code confirms this. So what happens to the <code>url</code> value?</p> <p><strong>UPDATE:</strong> It turns out that <code>data==None</code> so it was calling the first signature. This foiled my attempts to manually add the self parameter. As soon as I added the <code>=None</code> default to <code>data</code> in my <code>http_error_404</code> signature all was well (because it used the default).</p> <p>The <em>fixed</em> / <em>correct</em> signature is <code>def http_error_404(self, url, fp, errcode, errmsg, headers, data=None):</code></p>

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