[Python-ideas] Suggestions: dict.flow_update and dict.__add__

Jonathan Fine jfine2358 at gmail.com
Tue Mar 5 03:48:44 EST 2019

Instead of using dict + dict, perhaps use dict.flow_update. Here,
flow_update is just like update, except that it returns self.

There's a difference between a sorted copy of a list, and sorting the
list in place.

    >>> items = [2, 0, 1, 9]
    >>> sorted(items), items
    ([0, 1, 2, 9], [2, 0, 1, 9])
    >>> items.sort(), items
   (None, [0, 1, 2, 9])

In Python, mutating methods generally return None. Here, this prevents
beginners thinking their code has produced a sorted copy of a list,
when in fact it has done an in-place sort on the list. If they write
    >>> aaa = my_list.sort()
they'll get a None error when they use aaa.

The same goes for dict.update. This is a useful feature, particularly
for beginners. It helps them think clearly, and express themselves

This returning None can be a nuisance, sometimes. Suppose we have a
dictionary of default values, and a dictionary of use supplied
options. We wish to combine the two dictionaries, say into a new
combined dictionary.

One way to do this is:

   combined = defaults.copy()

But this is awkward when you're in the middle of calling a function:

          # lots of arguments, one to a line, with comments
          arg = combined, # Look up to see what combined is.
         # more arguments

There's a suggestion, that instead one extends Python so that this works:
        arg = defaults + options # What does '+' mean here?

USING flow_update
Here's another suggestion. Instead write:
        dict_arg = defaults.copy().flow_update(options) # Is this clearer?

Here's an implementation, as a subclass of dict.

    class mydict(dict):

        def flow_update(self, *argv, **kwargs):
            self.update(*argv, **kwargs)
            return self

        def copy(self):
            return self.__class__(self)

Not tested, using an assignment expression.
   dict_arg = (tmp := defaults.copy(), tmp.update(options))[0]
Not recommend.


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