Python: Check if one dictionary is a subset of another larger dictionary

Question

I\'m trying to write a custom filter method that takes an arbitrary number of kwargs and returns a list containing the elements of a database-like list that contain

Answer 1

This seemingly straightforward issue costs me a couple hours in research to find a 100% reliable solution, so I documented what I've found in this answer.

1. "Pythonic-ally" speaking, small_dict <= big_dict would be the most intuitive way, but too bad that it won't work. {'a': 1} < {'a': 1, 'b': 2} seemingly works in Python 2, but it is not reliable because the official documention explicitly calls it out. Go search "Outcomes other than equality are resolved consistently, but are not otherwise defined." in this section. Not to mention, comparing 2 dicts in Python 3 results in a TypeError exception.

2. The second most-intuitive thing is small.viewitems() <= big.viewitems() for Python 2.7 only, and small.items() <= big.items() for Python 3. But there is one caveat: it is potentially buggy. If your program could potentially be used on Python <=2.6, its d1.items() <= d2.items() are actually comparing 2 lists of tuples, without particular order, so the final result will be unreliable and it becomes a nasty bug in your program. I am not keen to write yet another implementation for Python<=2.6, but I still don't feel comfortable that my code comes with a known bug (even if it is on an unsupported platform). So I abandon this approach.

3. I settle down with @blubberdiblub 's answer (Credit goes to him):

   def is_subdict(small, big): return dict(big, **small) == big

   It is worth pointing out that, this answer relies on the == behavior between dicts, which is clearly defined in official document, hence should work in every Python version. Go search:

   • "Dictionaries compare equal if and only if they have the same (key, value) pairs." is the last sentence in this page
   • "Mappings (instances of dict) compare equal if and only if they have equal (key, value) pairs. Equality comparison of the keys and elements enforces reflexivity." in this page