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copied!<p>Edit: I have just realised I misread the question, and we are talking about a list of lists, not just a list. Updated to suit that.</p> <p>You can use a <a href="https://www.youtube.com/watch?v=t85uBptTDYY" rel="nofollow noreferrer">list comprehension</a> here to produce something a bit <a href="http://wiki.python.org/moin/PythonSpeed/PerformanceTips#Loops" rel="nofollow noreferrer">faster</a> and nicer to read:</p> <pre><code>def to_float_or_None(x): try: return float(x) except ValueError: return None database = [["1", "1.0", "", "foobar", "3.0"], ["1", "1.0", "", "foobar", "3.0"]] result = [[to_float_or_None(item) for item in record] for record in database] </code></pre> <p>Giving us:</p> <pre><code>[[1.0, 1.0, None, None, 3.0], [1.0, 1.0, None, None, 3.0]] </code></pre> <p>Edit: As noted by <a href="https://stackoverflow.com/users/63011/paolo-moretti">Paolo Moretti</a> in the comments, if you want the absolute fastest result, then using <code>map</code> <a href="https://stackoverflow.com/questions/1247486/python-list-comprehension-vs-map">may be faster</a> as we are not using a lambda function:</p> <pre><code>def to_float_or_None(x): try: return float(x) except ValueError: return None database = [["1", "1.0", "", "foobar", "3.0"], ["1", "1.0", "", "foobar", "3.0"]] result = [list(map(to_float_or_None, record)) for record in database] </code></pre> <p>Giving us the same result. I would note, however, that premature optimization is a bad thing. If you have identified this as a bottleneck in your application, then fair enough, but otherwise stick with the more readable over the fast.</p> <p>We still use a list comprehension for the outer loop as we would need a lambda function to use <code>map</code> again given it's dependence on <code>record</code>:</p> <pre><code>result = map(lambda record: map(to_float_or_None, record), database) </code></pre> <p>Naturally, if you want to evaluate these lazily, you can use generator expressions:</p> <pre><code>((to_float_or_None(item) for item in record) for record in database) </code></pre> <p>Or:</p> <pre><code>(map(to_float_or_None, record) for record in database) </code></pre> <p>This would be the preferred method unless you need the entire list at once.</p>

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