A “frozen” dictionary for Python

I wonder whether Raymond Hettinger has an opinion on this PEP. A long time ago, he wrote: "freezing dicts is a can of worms and not especially useful".

https://mail.python.org/pipermail/python-dev/2006-February/0...

I agree, same with frozenset. If you really want to use one of those as a key, convert to a tuple. There might be niche use cases for all this, but it's not something that the language or even the standard lib need to support.

Problem being that sets aren't consistently ordered and conversion to a tuple can result in an exponential (specifically, factorial) explosion in the number of possible keys associated with a single set. Nor can you sort all objects. Safe conversion of sets to tuples for use as keys is possible but the only technique I know requires an auxiliary store of objects (mapping objects to the order in which they were first observed), which doesn't parallelize well.

tuple(sorted(s)) and if you can't even sort the values, they're probably not hashable. I get that this involves a copy, but so does frozenset, and you can cross that bridge in various ways if it's ever a problem.

Here are some types that support hashing:

  str
  bytes
  int, float
  complex
  tuple
  frozenset

Aside from int and float, you cannot perform comparisons between objects of different types. Moreover, you cannot sort complex numbers at all.

I have crossed that bridge, and I'm telling you (again) that a sorted tuple is not a generic solution.