On Wed, Sep 28, 2016 at 12:54 AM, Jussi Piitulainen <jussi.piitulai...@helsinki.fi> wrote: > I wasn't sure if it makes a copy or just returns the dict. But it's > true: help(dict) says dict(mapping) is a "new dictionary initialized > from a mapping object's (key, value) pairs".
Yep. With mutable objects, Python's docs are usually pretty clear that you get a brand-new object every time: >>> x = [1,2,3,4] >>> y = list(x) >>> x == y True >>> x is y False >>> help(list) class list(object) | list() -> new empty list | list(iterable) -> new list initialized from iterable's items With immutables, the docs aren't always explicit, since by definition it can't matter. Sometimes they are, though: >>> x = 1,2,3 >>> tuple(x) is x True >>> help(tuple) class tuple(object) | tuple() -> empty tuple | tuple(iterable) -> tuple initialized from iterable's items | | If the argument is a tuple, the return value is the same object. Sometimes things get rather interesting. >>> help(int) class int(object) | int(x=0) -> integer | int(x, base=10) -> integer | | Convert a number or string to an integer, or return 0 if no arguments | are given. If x is a number, return x.__int__(). For floating point | numbers, this truncates towards zero. >>> class SubInt(int): ... def __int__(self): return self ... >>> x = SubInt(123) >>> x.__int__() is x True >>> int(x) is x False >>> type(int(x)) <class 'int'> Calling int(x) can return the exact object x, but only if x is an actual int, not a subclass. If it's a subclass, you get a base integer with the same value. In any case, mutables are generally going to be safe: every time you call the constructor, you get a new object. They won't try to cheat and return a reference to the same object. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
