Luke Lee

Software Engineer

Web + Desktop + Science

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Execution order of nested generator expressions

What does this print?

    units = [1, 2]
    tens = [10, 20]
    nums = (a + b for a in units for b in tens)
    units = [3, 4]
    tens = [30, 40]

I was surprised by the result because of the subtle way generator expressions are executed. The reasoning makes sense, which is to preserve your sanity when there's an error triggered by the generator.

This is a neat example of something I've always taken for granted. One could argue writing code like this is a bit obtuse and should be avoided. However, this is still an interesting detail to be aware of.

Check out the following articles for quiz-like questions and a discussion of surprising Python 'quirks.'

Published: 03-28-2014 13:51:20