Functions as values: passing one to another

A function is a value like any other. You can store it, put it in a dictionary and hand it to another function, and agent frameworks rely on all three.

def shout(text):
    return text.upper()

handler = shout
print(handler("refund please"))
print(shout)

shout without brackets is the function itself; with brackets, it is a call. handler = shout stores the function, so calling handler runs shout. The last line prints the function, not its result.

Sorting by a field

tickets = [
    {"id": 1, "priority": 2},
    {"id": 2, "priority": 5},
    {"id": 3, "priority": 3},
]

def by_priority(ticket):
    return ticket["priority"]

urgent_first = sorted(tickets, key=by_priority, reverse=True)
print([ticket["id"] for ticket in urgent_first])

sorted does not know how to order dictionaries. You pass it a function with key=, and it calls that function on each ticket and orders by what comes back. reverse=True puts the largest first.

lambda: a function without a name

urgent_first = sorted(tickets, key=lambda ticket: ticket["priority"], reverse=True)
print([ticket["id"] for ticket in urgent_first])

lambda ticket: ticket["priority"] is the same function as by_priority, written in place. It can hold one expression and no more, which is why it suits a key=.

Calling a function by its name

A model that uses tools does not run them. It answers with the name of a tool and its arguments, and the program looks the function up and calls it.

def lookup_order(order_id):
    return f"Order {order_id} shipped on 3 March."

def refund(order_id):
    return f"Refund started for order {order_id}."

tools = {"lookup_order": lookup_order, "refund": refund}

chosen = "refund"
print(tools[chosen]("A17"))

The dictionary maps names to functions. tools[chosen] is a function, and ("A17") calls it. Change chosen and a different function runs, with no if anywhere.