Functional Programming HOWTO — Python Documentation - The Diigo Meta page

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  Hardy Heron

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• 29 Jun 12
  

  annoyed
  • list comprehensions aren’t useful if you’re working with iterators that return an infinite stream or a very large amount of data. Generator expressions are preferable in these situations.
  • Generator expressions are surrounded by parentheses (“()”) and list comprehensions are surrounded by square brackets (“[]”). Generator expressions have the form:
  • Again, for a list comprehension only the outside brackets are different (square brackets instead of parentheses).

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• 18 Mar 10
  

  M Pagey

  • Two common operations on an iterator’s output are 1) performing some operation for every element, 2) selecting a subset of elements that meet some condition. For example, given a list of strings, you might want to strip off trailing whitespace from each line or extract all the strings containing a given substring.

     

    List comprehensions and generator expressions (short form: “listcomps” and “genexps”) are a concise notation for such operations, borrowed from the functional programming language Haskell (http://www.haskell.org).
  • but the parentheses signalling a function call also count
  • Any function containing a yield keyword is a generator function
  • Inside a generator function, the return statement can only be used without a value, and signals the end of the procession of values
  • I recommend that you always put parentheses around a yield expression when you’re doing something with the returned value, as in the above example. The parentheses aren’t always necessary, but it’s easier to always add them instead of having to remember when they’re needed.
  • Two of Python’s built-in functions, map() and filter(), are somewhat obsolete; they duplicate the features of list comprehensions but return actual lists instead of iterators
  • Which alternative is preferable? That’s a style question; my usual course is to avoid using lambda.

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