Rocco Moretti wrote: > Damien Wyart wrote: > >> * Efrat Regev <[EMAIL PROTECTED]> in comp.lang.python: >> >>> Suppose I have some non-numerical Foo and would like to create a list >>> of 20 Foo-s. Is there a one-step method (not a loop) of doing so? >> >> >> >> Maybe : >> >> [ Foo ] * 20 >> >> or, more verbose, >> >> [ Foo for _ in range(20) ] >> > > If Foo is mutable, keep this in mind: > > >>> a = [ [] ]*20 # A list of 20 empty lists > >>> print id(a[0]) == id(a[1]) > True > >>> a[0].append(1) > >>> print a > [[1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], > [1], [1], [1], [1], [1], [1]] > >>> > >>> b = [ [] for _ in range(20) ] # A list of 20 empty lists > >>> print id(b[0]) == id(b[1]) > False > >>> b[0].append(1) > >>> print b > [[1], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], > [], [], []] >
Oh, but also: >>> Foo = [] >>> c = [ Foo for _ in range(20) ] # A list of 20 empty lists >>> print id(c[0]) == id(c[1]) True >>> c[0].append(1) >>> print c [[1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1], [1]] (The difference with the 'b' case is that a new list is created each "time around", in the other cases, the same list is reused. -- http://mail.python.org/mailman/listinfo/python-list
