Python Map Function

Python map function is used to execute a function on all elements of an iterator. The function to execute will be passed to map() as a parameter. Through this blog post, we will understand map() function via many examples in coding.

A Quick Example:

def newfunc(val):
    return val * 2 
result = map(newfunc, [1, 2, 3, 4]) 
print(list(result))

# Result: [2, 4, 6, 8]

Lambda function likes a short hand of a loop.
(London – Source: Wallpapers Wide)

Definition and Usage

In python, map function syntax is quite simple:

map(function, iterators)

• function: Required and will execute for items of iterators
• iterators: Required. You can send many iterators. However, the function must have one parameter for each iterator.

Python Map Examples

Example 1: How to use lambda function with map()

In the above example, we can make it shorter, cleaner with a lambda function.

result = map(lambda x: x * 2, [1, 2, 3, 4]) 
print(list(result)) 

# Result: [2, 4, 6, 8]

If you are new to Python, you should read this article for more detail of lambda function (What is lambda function?)

Example 2: Passing multiple Iterators to map() function

Lambda function should have one parameter for each iterator.

iter1 = [1, 5, 7]
iter2 = [9, 5, 3]

result = map(lambda x, y: x + y, iter1, iter2)
print(list(result)) 

# Result: [10, 10, 10]

Example 3: Passing multiple Iterators with different size

Let's see what happen if we pass multiple iterators with different size.

In python 2, we got an exception. Therefore, be aware of this case in your code.

iter1 = [1, 5, 7, 9, 8]
iter2 = [9, 5, 3] 

result = map(lambda x, y: x + y, iter1, iter2) 
print(list(result)) 

"""
Result: 
   Traceback (most recent call last): File "main.py", line 6, in \<module\> result = map(lambda x, y: x + y, iter2, iter1) File "main.py", line 6, in \<lambda\> result = map(lambda x, y: x + y, iter2, iter1)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
"""

In python 3, we get a map object with the size of smallest iterator

iter1 = [1, 5, 7, 9, 8]
iter2 = [9, 5, 3]

result = map(lambda x, y: x + y, iter1, iter2)
print(list(result)) 

# Result: [10, 10, 10]

References

In this post, I provided some simple examples of the python map function. However, you can get more details from these links:

• Python Official Document
• Python 2 Official Document

The post Python Map Function appeared first on Python Geeks.

Comments

[Giora Guttsait]

Very cute! The first example also made me realise that map, filter, and the likes, that work on iterables, are lazy.

def newfunc(val):
  print('lazy ' + str(val)) # this is proof for lazy
  return val * 2

result = map(newfunc, [1, 2, 3, 4]) 
print(result) # prints the map object, but the newfunc operation
              # was not invoked on any of the values
print(next(result)) # only invokes newfunc on 1
print(list(result)) # invokes newfunc on the rest

repl.it example

[Python Geeks]

Awesome! It's an interesting thing that I haven't known yet. Thanks a lot for your sharing.
Btw, Don't you mind if I share your example in the original post? It will help other developers more understand map and filter.

[Giora Guttsait]

Also, notice that my code example has syntax highlighting.

This is because I use

```python
def some_method():
    pass
```

Instead of

```
def some_method():
    pass
```

[Python Geeks]

Thanks for the notice. It helps me a lot. :)

[Giora Guttsait]

I most certainly don't mind :)