75 – Match the structure of custom objects

75 – Match the structure of custom objects#

Custom objects can have their structure matched by specifying the relevant attribute names in the dunder attribute __match_args__.

This dunder attribute must be a tuple of strings that specifies the attribute names used for matching. The object can have more attributes, but the ones that aren’t included in __match_args__ are irrelevant for the purposes of structural pattern matching.

For example, the class Point below has three attributes but the dunder attribute __match_args__ only refers two:

class Point:
    __match_args__ = ("x", "y")

    def __init__(self, x, y, z):
        self.x, self.y = x, y
        self.z = z

p = Point(1, 2, 3)

When trying to match the structure of an instance of Point, you can match directly against the attributes x and y:

p = Point(1, 2, "")
match p:
    case Point(1, int()):
        print("match!")
# match!

Note how the case statement with the pattern Point(1, int()) looks like an instantiation of the class Point, although that is not valid code for the purposes of class instantiation. It is valid code for the purposes of structural pattern matching, though.