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From List Comprehensions to Generator Expressions

From List Comprehensions to Generator Expressions
List comprehensions were added in Python 2.0. This feature originated as a set of patches by Greg Ewing with contributions by Skip Montanaro and Thomas Wouters. (IIRC Tim Peters also strongly endorsed the idea.) Essentially, they are a Pythonic interpretation of a well-known notation for sets used by mathematicians. For example, it is commonly understood that this:

{x | x > 10}

refers to the set of all x such that x > 10. In math, this form implies a universal set that is understood by the reader (for example, the set of all reals, or the set of all integers, depending on the context). In Python, there is no concept of a universal set, and in Python 2.0, there were no sets. (Sets are an interesting story, of which more in a future blog post.)

This and other considerations led to the following notation in Python:

[f(x) for x in S if P(x)]

This produces a list containing the values of the sequence S selected by the predicate P and mapped by the function f. The if-clause is optional, and multiple for-clauses may be present, each with their own optional if-clause, to represent nested loops (the latter feature is rarely used though, since it typically maps a multi-dimensional entity to a one-dimensional list).


The history of python

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