Re: unpacking with default values
McA wrote: Hi all, probably a dumb question, but I didn't find something elegant for my problem so far. In perl you can unpack the element of a list to variables similar as in python (a, b, c = [0, 1, 2]), but the number of variables need not to fit the number of list elements. That means, if you have less list elements variables are filled with 'undef' (None in python), if you have more list elements as necessary the rest is ignored. How can I achieve this behaviour with python in an elegant and fast way? Best regards Andreas Mock -- http://mail.python.org/mailman/listinfo/python-list Python 3.0 has something a bit like this. Excess values can be bound (as a list) to the last variable: a,b,*c = [1,2,3,4,5] will result in c containing [3,4,5]. In Python 2.x, you can't do that directly, but you should be able to create a function that lengthens or shortens an input tuple of arguments to the correct length so you can do: a,c,b = fix(1,2) d,e,f = fix(1,2,3,4) However, the function won't know the length of the left hand side sequence, so it will have to be passed in as an extra parameter or hard coded. Gary Herron -- http://mail.python.org/mailman/listinfo/python-list
Re: unpacking with default values
On 17 Jul., 18:33, Gary Herron [EMAIL PROTECTED] wrote: In Python 2.x, you can't do that directly, but you should be able to create a function that lengthens or shortens an input tuple of arguments to the correct length so you can do: a,c,b = fix(1,2) d,e,f = fix(1,2,3,4) However, the function won't know the length of the left hand side sequence, so it will have to be passed in as an extra parameter or hard coded. Hi Gary, thank you for the answer. Do you know the protocol used by python while unpacking? Is it a direct assingnment? Or iterating? Best regards Andreas Mock -- http://mail.python.org/mailman/listinfo/python-list
Re: unpacking with default values
McA wrote: On 17 Jul., 18:33, Gary Herron [EMAIL PROTECTED] wrote: In Python 2.x, you can't do that directly, but you should be able to create a function that lengthens or shortens an input tuple of arguments to the correct length so you can do: a,c,b = fix(1,2) d,e,f = fix(1,2,3,4) However, the function won't know the length of the left hand side sequence, so it will have to be passed in as an extra parameter or hard coded. Hi Gary, thank you for the answer. Do you know the protocol used by python while unpacking? Is it a direct assingnment? Or iterating? Both I think, but what do you mean by *direct* assignment? Best regards Andreas Mock -- http://mail.python.org/mailman/listinfo/python-list It RHS of such an assignment can be any iterable (I think). Lets test: (The nice thing about an interactive Python session, it that it's really easy to test.) L = [1,2,3] a,b,c=L a,b=L Traceback (most recent call last): File stdin, line 1, in module ValueError: too many values to unpack a,b,c,d=L Traceback (most recent call last): File stdin, line 1, in module ValueError: need more than 3 values to unpack G = (f for f in [1,2,3]) # A generator expression a,b,c = G G = (f for f in [1,2,3]) # A generator expression a,b = G Traceback (most recent call last): File stdin, line 1, in module ValueError: too many values to unpack G = (f for f in [1,2,3]) # A generator expression a,b,c,d = G Traceback (most recent call last): File stdin, line 1, in module ValueError: need more than 3 values to unpack I'd call that direct assignment with values supplied by any iterable. Gary Herron -- http://mail.python.org/mailman/listinfo/python-list
Re: unpacking with default values
McA wrote: Do you know the protocol used by python while unpacking? Is it a direct assingnment? Or iterating? In CPython, at least, both, just as with normal unpack and multiple assignment. The iterable is unpacked into pieces by iterating (with knowledge of the number of targets and which is the catchall). The targets are then directly bound. from dis import dis # first standard assignment dis(compile(a,b,c = range(3), '','single')) 1 0 LOAD_NAME0 (range) 3 LOAD_CONST 0 (3) 6 CALL_FUNCTION1 9 UNPACK_SEQUENCE 3 12 STORE_NAME 1 (a) 15 STORE_NAME 2 (b) 18 STORE_NAME 3 (c) 21 LOAD_CONST 1 (None) 24 RETURN_VALUE # now starred assignment dis(compile(a,b,*c = range(3), '','single')) 1 0 LOAD_NAME0 (range) 3 LOAD_CONST 0 (3) 6 CALL_FUNCTION1 9 UNPACK_EX2 12 STORE_NAME 1 (a) 15 STORE_NAME 2 (b) 18 STORE_NAME 3 (c) 21 LOAD_CONST 1 (None) 24 RETURN_VALUE The only difference is UNPACK_EX (tended) instead of UNPACK_SEQUENCE. Tne UNPACK_EX code is not yet in the dis module documentation. But a little reverse engineering reveals the parameter meaning: a,*b,c and *a,b,c give parameters 257 and 512 instead of 2. Separating 2,257,512 into bytes gives 0,2; 1,1; 2,0. a,*b and *a,b give 1, 256 or 0,1; 1,0. The two bytes are the number of targets after and before the starred target. Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
