On what basis do you ascertain whether "==" would work correctly? Please explain.
Hi Chris, I'm just glancing at the line of code, and doing a little thought experiment to see if it would get the same output if == was used instead. For a singleton like None or False or the class like "list" .. == will return the same answer as "is". Look at these lines; if mode is None: if type(items) is list: If that code works with "is" it's going to work with == too. People are not used to seeing == in these cases, but it works:
x = None x is None True x == None True
t = type([1, 2, 3]) t is list True t == list True
fn = list.index fn is list.index True fn == list.index True
The situations where "is" is truly needed are rather esoteric. Best, Nick On Mon, Aug 30, 2021 at 2:02 PM Chris Angelico <rosuav@gmail.com> wrote:
On Tue, Aug 31, 2021 at 6:52 AM Nick Parlante <nick@cs.stanford.edu> wrote:
I claimed that uses of "is" where it is needed for correctness are quite rare. Let me back that up with a little data here.
Just as a random supply of Python code, let's look at the first four Python modules where the name starts with a letter from the Python standard modules list https://docs.python.org/3/py-modindex.html : abc.py aifc.py argparse.py ast.py (The array module appears to be in C)
Strip out PyDoc and string literals and just grep for " is " (code included below if you'd like to try it yourself). Look at those lines - how many of those uses of "is" are needed for correctness, where the "is" is really doing something, and how many would work fine with ==? The resulting lines of code are included below.
There's about 90 uses of is/is-not in this sample. 100% of these uses would work correctly using ==. Not a single one of these uses actually relies on the "is" computation for correctness.
On what basis do you ascertain whether "==" would work correctly? Please explain.
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