On Fri, Sep 8, 2017 at 2:24 PM, Stefan Ram <r...@zedat.fu-berlin.de> wrote: > Maybe you all know this, but to me this is something new. > I learnt it by trial and error in the Python 3.6.0 console. > > Most will know list comprehensions: > > |>>> [ i for i in range( 3, 5 )] > |[3, 4] > > I found out that the comprehension can be detached from the list: > > |>>> k =( i for i in range( 3, 5 )) > > but one must use an extra pair of parentheses around it in the > assignment.
You don't need an *extra* pair of parentheses per se. The generator expression simply must be enclosed in parentheses. This is perfectly valid: py> sum(i for i in range(3, 5)) > Now I can insert the "generator" »k« into a function call, > but a spread operator should cannot be used there. If you mean the * syntax, certainly it can. > |>>> sum( k ) > |7 But you don't want it in this case, because that would do the wrong thing. > »sum« expects exactly two arguments, and this is what »k« > provides. Not exactly. sum(k) is only providing the first argument to sum. The first argument is required to be an iterable, which is what k is. The second argument, "start", takes its default value of 0. This is effectively equivalent to calling sum([3, 4]). sum(*k) on the other hand would spread the values of k over the arguments of sum and would be equivalent to calling sum(3, 4), which raises a TypeError because 3 is not iterable. > But to insert it again into the place where it was "taken > from", a spread operator is required! > > |>>> k =( i for i in range( 3, 5 )) > |>>> [ *k ] > |[3, 4] Alternatively: py> list(k) [3, 4] As a final aside, storing a generator in a temporary variable is usually bad style because it sets up an expectation that the contents of the variable could be used more than once, when in fact a generator can only be iterated over once. -- https://mail.python.org/mailman/listinfo/python-list
