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    <p>Your case is a particular case: <strong>'newtext'</strong> has exactly the same number of characters as <strong>'oldtext'</strong>. </p> <p>Hence, it is possible to use one of the following codes to replace exactly a word <strong>'oldtext'</strong> or a line in which the word <strong>'oldtext'</strong> is present, by word <strong>'newtext'</strong> or a line in which <strong>'newtext'</strong> replaces <strong>'oldtext'</strong>.</p> <p>.</p> <p>If files have not super-big size, the content of each file can be read entirely into memory:</p> <pre><code>from os import fsync # code using find() count = 0 for match in all_files('*.html', '.'): with open(match,'rb+') as thefile: diag = False fno = thefile.fileno() content = thefile.read() thefile.seek(0,0) x = content.find('oldtext') while x&gt;=0: diag = True thefile.seek(x,1) thefile.write('newtext') thefile.flush() fsync(fno) x = content[thefile.tell():].find('oldtext') if diag: cnt += 1 </code></pre> <p>or</p> <pre><code>from os import fsync # code using a regex import re pat = re.compile('oldtext') count = 0 for match in all_files('*.html', '.'): with open(match,'rb+') as thefile: diag = False fno = thefile.fileno() content = thefile.read() thefile.seek(0,0) prec = 0 for mat in pat.finditer(content): diag = True thefile.seek(mat.start()-prec,1) thefile.write('newtext') thefile.flush() fsync(fno) prec = mat.end() if diag: cnt += 1 </code></pre> <p>.</p> <p>For heavy files, a reading and rewriting line after line is possible:</p> <pre><code>from os import fsync # code for big files, using regex import re pat = re.compile('oldtext') count = 0 for match in all_files('*.html', '.'): with open(match,'rb+') as thefile: diag = False fno = thefile.fileno() line = thefile.readline() while line: if 'oldtext' in line: diag = True thefile.seek(-len(line),1) thefile.write(pat.sub('newtext',line)) thefile.flush() fsync(fno) line = thefile.readline() if diag: cnt += 1 </code></pre> <p>.</p> <p>The instructions <code>thefile.flush()</code> and <code>fsync(fno)</code> are required after each writing in order that the file handler <code>thefile</code> points with accuracy on the exact position in the file at any moment. They allow to obtain effective writing ordered by instuction <code>write()</code> </p> <blockquote> <p>flush() does not necessarily write the file’s data to disk. Use flush() followed by os.fsync() to ensure this behavior. <a href="http://docs.python.org/library/stdtypes.html#file.flush" rel="nofollow">http://docs.python.org/library/stdtypes.html#file.flush</a></p> </blockquote> <p>.</p> <p>These programs do the minimum. So I think they are fast.</p> <p>.</p> <p>Nota bene <strong>:</strong> a file opened in mode <code>'rb+'</code> have no changing of its time of last modification if no modification has been performed.</p>
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    1. POPython string replace in a file without touching the file if no substitution was made
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    1. COThanks for the detailed answer. Also, I did not know about the file mode 'rb+' so that is a good tip. My question was actually more general: I want to replace the occurrence of a given string with a different given string.
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    2. CO@LandedGently If your problem is _"ACTUALLY more general"_, WHY did you write _"I'm trying to replace occurrences of 'oldtext' with 'newtext'"_ in your question ? However, it's true that in the codes in your question, there are instructions like that: ``"replace(oldtext, newtext)"`` in which ``oldtext`` and ``newtext`` are not string values but names of objects.
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    3. CO@LandedGently It's precisely because these instructions are contradictory with the phrasing _"occurrences of 'oldtext' with 'newtext'"_ that I asked you : _"Do you mean that you want to replace all the strings 'oldtext' present in a file with strings 'newtext' ? Or that you want to replace the occurences of the string hold by the object of name oldtext with the string hold by the object of name newtext ?"_
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