How to match an empty dictionary?

Question

Python supports Structural Pattern Matching since version 3.10. I came to notice that matching an empty dict doesn't work by simply matching {} as it does for lists. According to my naive approach, non-empty dicts are also matched (Python 3.10.4):

def match_empty(m):
    match m:
        case []:
            print("empty list")
        case {}:
            print("empty dict")
        case _:
            print("not empty")

match_empty([])           # empty list
match_empty([1, 2])       # not empty
match_empty({})           # empty dict
match_empty({'a': 1})     # empty dict

Matching the constructors even breaks the empty list matching:

def match_empty(m):
    match m:
        case list():
            print("empty list")
        case dict():
            print("empty dict")
        case _:
            print("not empty")

match_empty([])           # empty list
match_empty([1, 2])       # empty list
match_empty({})           # empty dict
match_empty({'a': 1})     # empty dict

Here is a solution, that works as I expect:

def match_empty(m):
    match m:
        case []:
            print("empty list")
        case d:
            if isinstance(d, dict) and len(d) == 0:
                print("empty dict")
                return
            print("not empty")

match_empty([])           # empty list
match_empty([1, 2])       # not empty
match_empty({})           # empty dict
match_empty({'a': 1})     # not empty

Now my questions are:

• Why do my first 2 approaches not work (as expected)?
• Is there a way to use structural pattern matching to match only an empty dict (without checking the dict length explicitly)?

Answer 1

Using a mapping (dict) as the match pattern works a bit differently than using a sequence (list). You can match the dict's structure by key-value pairs where the key is a literal and the value can be a capture pattern so it is used in the case.

You can use **rest within a mapping pattern to capture additional keys in the subject. The main difference with lists is - "extra keys in the subject will be ignored while matching".

So when you use {} as a case, what you're really telling Python is "match a dict, no constraints whatsoever", and not "match an empty dict". So one way that might be slightly more elegant than your last attempt is:

def match_empty(m):
    match m:
        case []:
            print("empty list")
        case {**keys}:
            if keys:
                print("non-empty dict")
            else:
                print("empty dict")
        case _:
            print("not empty")

---

I think the main reason this feels awkward and doesn't work good is because the feature wasn't intended to work with mixed types like this. i.e. you're using the feature as a type-checker first and then as pattern matching. If you knew that m is a dict (and wanted to match its "insides"), this would work much nicer.