2.1 Lambda Expressions
Lambda expressions are expressions that evaluate to functions by specifying two things: the parameters and a return expression.
lambda <parameters>: <return expression>
While both lambda expressions and def statements create function objects, there are some notable differences. lambda expressions work like other expressions; much like a mathematical expression just evaluates to a number and does not alter the current environment, a lambda expression evaluates to a function without changing the current environment. Let's take a closer look.
| lambda | def | |
|---|---|---|
| Type | Expression that evaluates to a value | Statement that alters the environment |
| Result of execution | Creates an anonymous lambda function with no intrinsic name. | Creates a function with an intrinsic name and binds it to that name in the current environment. |
| Effect on the environment | Evaluating a lambda expression does not create or modify any variables. | Executing a def statement both creates a new function object and binds it to a name in the current environment. |
| Usage | A lambda expression can be used anywhere that expects an expression, such as in an assignment statement or as the operator or operand to a call expression. | After executing a def statement, the created function is bound to a name. You should use this name to refer to the function anywhere that expects an expression. |
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lambda example
# A lambda expression by itself does not alter # the environment lambda x: x * x # We can assign lambda functions to a name # with an assignment statement square = lambda x: x * x square(3) # Lambda expressions can be used as an operator # or operand negate = lambda f, x: -f(x) negate(lambda x: x * x, 3) # We can directly call a lambda expression # just created # Make sure the lambda expression wrapped in a # pair of parenthesis or between `(`, `,`, and `)` # in order not to let Python misunderstand you (lambda x: x * 2 ** x)(5) # evaluates to 160 -
def example
def square(x): return x * x # A function created by a def statement # can be referred to by its intrinsic name square(3)