Is there a description of how __cmp__ works for dict objects in Python 2?

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If you are asking how comparing dictionaries works, it is this:

  • To compare dicts A and B, first compare their lengths. If they are unequal, then return cmp(len(A), len(B)).
  • Next, find the key adiff in A that is the smallest key for which adiff not in B or A[adiff] != B[adiff]. (If there is no such key, the dicts are equal.)
  • Also find the smallest key bdiff in B for which bdiff not in A or A[bdiff] != B[bdiff].
  • If adiff != bdiff, then return cmp(adiff, bdiff). Else return cmp(A[adiff], B[bdiff]).

In pseudo-code:

def smallest_diff_key(A, B):
    """return the smallest key adiff in A such that adiff not in B or A[adiff] != B[bdiff]"""
    diff_keys = [k for k in A if k not in B or A[k] != B[k]]
    return min(diff_keys)

def dict_cmp(A, B):
    if len(A) != len(B):
        return cmp(len(A), len(B))
    try:
        adiff = smallest_diff_key(A, B)
    except ValueError:
        # No difference.
        return 0
    bdiff = smallest_diff_key(B, A)
    if adiff != bdiff:
        return cmp(adiff, bdiff)
    return cmp(A[adiff], b[bdiff])

This is translated from the 2.6.3 implementation in dictobject.c.

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