Hello, This is an idea about something I'd like to see implemented in python. I understand that's the purpose of PEPs, so I'll write it as a PEP, but send it here to receive your valuable feedback.
Abstract This is a proposal to increase the richness of for loops, only to the extent that it equals that of list and generator comprehensions. In the opinion of the proponent, this will make the language more uniform and would reduce the excessive level of nesting that is required sometimes, without introducing special keywords, or breaking backwards compatibility. This PEP is independent of, but related to PEP 3142. Rationale The syntax of a for loop is restricted to the following: >>> for element in list: >>> instructions Nesting for loops and conditionals is possible, of course: >>> for x in list1: >>> for y in list2: >>> if condition(x,y): >>> func2(x,y) However, for list and generator comprehensions, the syntax is more concise: >>> list=[func(x,y,..) for x in list1 for y in list2 ... if condition(x,y,...)] Loops and comprehensions serve for similar purposes, and sometimes what was first written in one way is finally changed into the other. Thus it would be convenient to allow for more similar syntax in both of them. This requires that the syntax of a for loop allows for nested for and conditionals (and possibly while loops (see PEP 3142)) >>>for x in list1 for y in list2 ... if condition(x,y,...): >>> ''instructions'' In general, whenever >>> [expression1 iterator] is a valid list comprehension, then >>> iterator: >>> statements should be a valid loop. As another example, it is sometimes the case that an iteration over two lists can be rewritten as a single iteration: >>> for x in range(10) for y in range(10) if x+y==5: >>> print x,y is equivalent to >>> for x in range(6): >>> print x,5-y it is somewhat more conceptual to think of the first iteration as a single loop. It is actually possible to come very close to the notation proposed using the generator comprehensions introduced in python 2.5: >>> for x,y in ((x,y) for x in range(10) for y in range(10) if x+y==5): >>> print x,y but this notation is cumbersome. This PEP responds to the philosophy that if a nesting level is unused, it should be avoided. For example, in the code: >>> for x in list1: >>> func1(x) >>> for y in list2: >>> if cond(x,y): >>> func(x,y) >>> func3(x) the second nesting level doesn't contain any statements and thus, should this PEP be passed, it should be written: >>> for x in list1: >>> func1(x) >>> for y in list2 if cond(x,y): >>> func(x,y) >>> func3(x) -- http://mail.python.org/mailman/listinfo/python-list
