Python Function – lambda

Lambda is a single line anonymous function used in python.  The format is like below:

lambda parameters: expression

sample code

>>> (lambda x: x*x)(3)

9

paramters are where you put your parameter list and expression is the function expression you want to return.

Some points:

• You don’t need to use () for the parameter list.
• No explicit return statement
• Lambda expression can be used as a function by (lambda) (parameters)

Here is another example

>>> Condition = True

>>> ObjectString = "my_email_address gmail.com"

>>> processFunc = Condition and (lambda s: "@".join(s.split())) or (lambda s: s)

>>> processFunc(ObjectString)

'my_email_address@gmail.com'

processFunc is actually a function with a Condition.

 

In real word, lambda is widely used with the build-in functions such as map, filter and reduce.  

Get the square of each element in the list

mylist = [1,3,5,7,9,0]
map( lambda x: x*x , mylist)  
or it can be expressed in list comprehension
[ x*x for x in mylist]
 

Get all the element if the element is bigger than 0

mylist = [1,-2,3,-4,5,-6,7,-8]
Filter((lambda x: x> 0), mylist)

Dictionary sort

sorted(dict.items(), key=lambda e:e[1], reverse=True) – sort key by value
sorted(dict.items(), key=lambda e:e[0], reverse=True) – sort key by key