Python lambda function missing lambda keyword but still works

Question

I am using python library tinydb to store data.

According to tinydb documentation, the proper syntax for an example query is:

User = Query()
db.search(User.birthday.year == 1990)

Why don't we need:

User = Query()
db.search(lambda User: User.birthday.year == 1990)

db.search is a function that is only called once. This means that the function is receiving a fixed value (the result of a comparison) as an input, not a function to serve as a comparator?

How does the tinydb library achieve this weird syntax?

chepner · Accepted Answer · 2019-03-20 14:40:05Z

4

User.birthday.year isn't really an int; it's an object that represents a query to retrieve an int. The type of this object likewise defines __eq__ to return not a Boolean value, but a function that will return the result of comparing the fetched int to 1990.

Digging into the code confirms this.

Query.__getattr__ returns a new Query object, so User.birthday.year is also a Query.

The definitions of Query.__eq__ and Query._generate_test confirm that == also builds a new query.

edited Mar 20, 2019 at 14:40

answered Mar 20, 2019 at 14:17

chepner

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2 Comments

Michael Sohnen Over a year ago

So are you saying that the way it achieves this is by using __eq__ dunder magic to (essentially) override/overload the "==" operator?

chepner Over a year ago

Not just essentially; __eq__ defines what == means for all types. If you don't override it, you inherit its definition from a parent class, which often means going back to object.__eq__, which simply checks for object identity.

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