Learning Python

Odds and Ends

So far, we’ve seen what it takes to write our own functions in Python. There are a handful of additional function-related ideas we’d like to introduce in this section:

• lambda creates anonymous functions.

• apply calls functions with argument tuples.

• map runs a function over a sequence and collects results.

• Functions return None if they don’t use a return statement.

• Functions present design choices.

• Functions are objects, just like numbers and strings.

lambda Expressions

Besides the def statement, Python also provides an expression form that generates function objects. Because of its similarity to a tool in the LISP language, it’s called lambda. Its general form is the keyword lambda, followed by one or more arguments, followed by an expression after a colon:

                  
                  lambda argument1, argument2,... argumentN : Expression using arguments

Function objects returned by lambda expressions are exactly the same as those created and assigned by def. But the lambda has a few differences that make it useful in specialized roles:

lambda is an expression, not a statement

Because of this, a lambda can appear in places a def can’t—inside a list constant, for example. As an expression, the lambda returns a value (a new function), which can be assigned a name optionally; the def statement always assigns the new function to the name in the header, instead of returning it as a result.

lambda bodies are a single expression, not a block of statements

The lambda’s body is similar to what you’d put in a ...