how to detect if a dictionary has been modified ?

bearophileHUGS at bearophileHUGS at
Sun Nov 23 12:47:09 CET 2008

Paul Rubin:
> As a couple of people have said, you could make a dict subclass that
> notices when you do updates.  I think the real answer is a
> "functional" dictionary, which means one that is never updated.

Python V. 2.4, 2.5, 2.6. 3.0 have gained more and more lazy-style
features, this is changing a little how normal Python programs are
written (compared for example with Haskell, using lazy computations in
Python is less handy and probably less safe too, because for example
in Haskell you can define something that lazily generates an infinite
sequence of items, this is now easy to do in Python too, but in
Haskell inside the generator you can use items generated in the past
too, and the generators don't exhaust).

In the last years functional languages are raising their elegant
lambda-shaped heads more and more, so today you can find several
immutable data structures in Scala and F# languages (and in the D
language true closures, simple ways to state recursively immutable
things, and even pure functions. This will make D like a cleaned up,
simplified and improved C++ with functional features added. Then D3
may add AST macros. At that point pattern-matching programming,
dataflow programming, constraint programming and Logic programming
become the few programming styles not supported natively).

So maybe in the following years Python too may gain few other
immutable data structures, fitter for a functional style of
programming (in Python there are already some immutable data
structures, like strings, tuples, frozensets, and I sometimes use
frozendicts. Time objects and time deltas too are often good kept as


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