CODEWITHSUNDEEP
pythonpython-2.7set
TRX
TRX

Reputation: 101

Why can I not compare a set and immutableset in python 2.7

Why is it not possible to compare a set and ImmutableSet using the subset operator <= ? E.g. run the following code. What's the problem here? Any help appreciated. I'm using Python 2.7.

>>> from sets import ImmutableSet
>>> X = ImmutableSet([1,2,3])
>>> X <= set([1,2,3,4])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sets.py", line 291, in issubset
    self._binary_sanity_check(other)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sets.py", line 328, in _binary_sanity_check
    raise TypeError, "Binary operation only permitted between sets"
TypeError: Binary operation only permitted between sets
>>>

Upvotes: 2

Views: 936

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1122222

Use a frozenset object instead; the sets module has been deprecated and is not comparable with the built-in types:

>>> X = frozenset([1,2,3])
>>> X <= set([1,2,3,4])
True

From the documentation for the sets module:

Deprecated since version 2.6: The built-in set/frozenset types replace this module.

If you are stuck with code using the sets module, stick to its types exclusively when comparing:

>>> from sets import Set, ImmutableSet
>>> Set([1, 2, 3]) <= set([1, 2, 3, 4])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/Libraries/buildout.python/parts/opt/lib/python2.7/sets.py", line 291, in issubset
    self._binary_sanity_check(other)
  File "/Users/mj/Development/Libraries/buildout.python/parts/opt/lib/python2.7/sets.py", line 328, in _binary_sanity_check
    raise TypeError, "Binary operation only permitted between sets"
TypeError: Binary operation only permitted between sets
>>> ImmutableSet([1, 2, 3]) <= Set([1, 2, 3, 4])
True

Python set and frozenset do accept any sequence for many of the operators and functions, so inverting your test also works:

>>> X
frozenset([1, 2, 3])
>>> set([1,2,3,4]) >= X
True

The same applies to the .issubset() function on the sets.ImmutableSet and sets.Set classes:

>>> X.issubset(set([1,2,3,4]))
True

but not mixing the deprecated types and the new built-ins is entirely the best option.

Upvotes: 7

Related Questions