Python Lambda

I saw this while reading someone else’s code and realized I wasn’t very familiar with it, so I’m reviewing and taking notes.

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.

Syntax

lambda arguments : expression

Executes the expression and returns the result.

Example

x = lambda a : a + 10
print(x(5))

This single line defines a lambda function, where a is the parameter, a+10 is the expression, and x is the function name.

Lambda can accept any number of arguments, for example:

x = lambda a, b, c : a + b + c
print(x(5, 6, 2))

This function takes three parameters.

Anonymous Functions Inside Functions

Suppose I define a function like this:

def myfunc(n):
  return lambda a : a * n

This function multiplies a by n.

def myfunc(n):
  return lambda a : a * n

mydoubler = myfunc(2)

This quickly builds a function like:

def mydoubler(a):
  return n*2

No need to define another function when you want something else. For example, if I want a tripler function, I can just do:

mytripler = myfunc(3)

zip

The zip() function takes iterable objects as arguments, packs corresponding elements from the objects into tuples, and returns an object composed of these tuples. This saves a lot of memory.

We can use list() to convert and output a list.

If the iterators have different numbers of elements, the returned list length matches the shortest object. Using the * operator, you can unzip tuples into lists.

Syntax

zip([iterable, ...])

Example

>>>a = [1,2,3]
>>> b = [4,5,6]
>>> c = [4,5,6,7,8]
>>> zipped = zip(a,b)     # Returns an object
>>> zipped
<zip object at 0x103abc288>
>>> list(zipped)  # list() converts to list
[(1, 4), (2, 5), (3, 6)]
>>> list(zip(a,c))# Number of elements matches shortest list
[(1, 4), (2, 5), (3, 6)]
>>> a1, a2 = zip(*zip(a,b))# Opposite of zip, zip(*) can be understood as unzip
>>> list(a1)
[1, 2, 3]
>>> list(a2)
[4, 5, 6]

map

map() applies a provided function to map over a specified sequence.

The first parameter function calls the function on each element in the parameter sequence, returning a new list containing the return values of each function call.

Syntax

map(function, iterable, ...)

Example

>>>def square(x) :            # Calculate square
return x ** 2
>>> map(square, [1,2,3,4,5])   # Calculate square of each element
[1, 4, 9, 16, 25]
>>> map(lambda x: x ** 2, [1, 2, 3, 4, 5])  # Using lambda anonymous function
[1, 4, 9, 16, 25]  # Provided two lists, adds elements at same positions
>>> map(lambda x, y: x + y, [1, 3, 5, 7, 9], [2, 4, 6, 8, 10])
[3, 7, 11, 15, 19]

dict

A dictionary is another mutable container model that can store objects of any type.

Each key-value pair key=>value in a dictionary is separated by a colon :, each pair is separated by a comma ,, and the entire dictionary is enclosed in curly braces {}. The format is:

d = {key1 : value1, key2 : value2}