In many cases, the definition of a helper function requires too much code. Often, we can digest the key function to a single expression. It can seem wasteful to have to write both def and return statements to wrap a single expression.

Python offers the lambda form as a way to simplify using higher-order functions. A lambda form allows us to define a small, anonymous function. The function's body is limited to a single expression.

The following is an example of using a simple lambda expression as the key:

long, short = max(trip, key=lambda leg: leg[2]), min(trip, key=lambda leg: leg[2])
print(long, short)

The lambda we've used will be given an item from the sequence; in this case, each leg three tuple will be given to the lambda. The lambda argument variable, leg, is assigned and the expression, leg[2], is evaluated, plucking the distance from the three tuple.

In the rare case that a lambda is never reused, this form is ideal. It's common, however, to need to reuse...