Python `filter()`

function is used to remove or filter out some items from an iterable based on a condition.

`filter()`

accepts two arguments and returns another iterable which contains the elements which were not filtered or which satisfied the condition.

**filter() syntax**

Syntax of `filter()`

function is

iterable = filter(function, iterable)

`filter()`

accepts two arguments.

1. A function which returns `True`

or `False`

.

2. An iterable. This iterable may be a **string**, **list**, set or a **tuple**.

Function given as argument to `filter()`

is invoked for* each element of iterable*.

If the function returns `True`

, this element is included in the result otherwise it is ignored.

`filter()`

returns an object of class `filter`

which is an iterable.

**filter() example**

Suppose there is a list of numbers and you want to create a new list from the first list having only those numbers which are greater than 0.

Python code without `filter()`

would be

# list of numbers numbers = [10,-2, 57, -9, 12,-34] # list of positive numbers positive = [] # iterate over list for num in numbers: # check if number is greater than 0 if num >= 0: positive.append(num) print('Original list:', numbers) print('Numbers greater than 0:', positive)

This prints

Original list: [10, -2, 57, -9, 12, -34] Numbers greater than 0: [10, 57, 12]

Above example can be modified to use `filter()`

function as below

def check_positive(num): return num >= 0 # list of numbers numbers = [10,-2, 57, -9, 12,-34] filtered_object = filter(check_positive, numbers) # list of positive numbers positive = list(filtered_object) print('Original list:', numbers) print('Numbers greater than 0:', positive)

**Explanation**

Above code defines a function `check_positive`

which returns `True`

or `False`

as the argument it receives is greater than or lesser than 0 respectively.

This function is supplied to `filter()`

as the first argument. Second argument to `filter()`

is a list of numbers.

For each element of the list `check_positive`

is invoked. If this function returns `True`

, the corresponding element is added to the result otherwise not.

Note that there are no parenthesis around function name in `filter()`

and each list element is sent to the function implicitly.

`filter()`

returns an object of class `filter`

which is then passed on to list constructor to create a new list.

Instead of assigning `filter()`

result to a new variable, it can be directly supplied to list constructor as

positive = list(filter(check_positive, numbers))

**Filter with lambda function**

Lambda functions are functions without a name also referred as anonymous functions.

**Lambda functions**are defined at the location where they are required using

`lambda`

keyword.As we saw above,

`filter()`

accepts a function as argument. Instead of creating a **separate function**, we can write it as a lambda function.

Thus, above example can be modified as

# list of numbers numbers = [10,-2, 57, -9, 12,-34] filtered_object = filter(lambda num: num >= 0, numbers) # list of positive numbers positive = list(filtered_object) print('Original list:', numbers) print('Numbers greater than 0:', positive)

This will get the same result. Notice how `filter()`

is used with a lambda function.

**filter() without argument function**

`filter()`

function can also be called without the argument function or writing `None`

in place of argument function.

If `None`

is used as argument function, it is called **Identity function** and in that case, values which result in `True`

are added to the result.

**Example,**

numbers = [10, -2, 57, -9, 12,-34, 1, 0] filtered_object = filter(None, numbers) # list of positive numbers positive = list(filtered_object) print('Original list:', numbers) print('Numbers greater than 0:', positive)

This prints

Original list: [10, -2, 57, -9, 12, -34, 1, 0] Numbers greater than 0: [10, -2, 57, -9, 12, -34, 1]

Notice that 0 is removed from the result since it is considered as `False`

. All other numbers and 1 are assumed to be `True`

.

If the list contains a boolean `False`

value, then it will also be removed by Identity function.

`filter()`

function in python.
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