[Tut] Python List count()

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Python List count()

This tutorial shows you everything you need to know to help you master the essential

Code:

count()

method of the most fundamental container data type in the Python programming language.



Definition and Usage:

The

Code:

list.count(x)

method counts the number of occurrences of the element

Code:

x

in the

Code:

list

.

Here’s a short example:

Code:

>>> lst = [1, 2, 42, 2, 1, 42, 42] >>> lst.count(42) 3 >>> lst.count(2) 2

In the first line of the example, you create the list

Code:

lst

. You then count the number of times the integer values 42 and 2 appear in the list.

Code Puzzle — Try It Yourself:

Now you know the basics. Let’s deepen your understanding with a short code puzzle—can you solve it?

https://repl.it/@finxter/ListCount?lite=true

You can also solve this puzzle and track your Python skills on our interactive Finxter app.

Syntax: You can call this method on each list object in Python. Here’s the syntax:

Code:

list.count(value)

Arguments:

Argument |
Description

Code:

value

|
Counts the number of occurrences of

Code:

value

in

Code:

list

. A value appears in the list if the

Code:

==

operator returns

Code:

True

.

Return value: The method

Code:

list.count(value)

returns an integer value set to the number of times the argument

Code:

value

appears in the list. If the value does not appear in the list, the return value is 0.

Python List Count Values

Python List Count Duplicates

Python List Count Unique Values and Strings

Python List Count All Elements

Python List Count Lambda

Python List Count With Condition

Python List Count If

Python List Count Greater Than

Python List Count Smaller Than

Python List Count Regex

Python List Count Matches

Python List Count Wildcard

Python List Count Not Working

Python List Count Runtime Complexity

Python List Reference Count

Python List Count to Dict

Python List Count Tuples

Python List Count Zero / Non-Zero

Python List Count and Sort

Python List Count Slow

Python List Count Group By

Python List Count vs Len

Python List reverse() Time Complexity

The time complexity of the

Code:

reverse()

operation is O(n) for a list with n elements. The standard Python implementation cPython“touches” all elements in the original list to move them to another position. Thus, the time complexity is linear in the number of list elements.

You can see a plot of the time complexity of the

Code:

reverse()

method for growing list size here:



The figure shows how the elapsed time of reversing lists with growing number of elements grows linear to the number of elements.

If you’re interested in the code I used to generate this plot with Matplotlib, this is it:

Code:

import matplotlib.pyplot as plt import time y = [] for i in [100000 * j for j in range(10,100)]: lst = list(range(i)) t0 = time.time() x = lst.reverse() t1 = time.time() y.append(t1-t0) plt.plot(y) plt.xlabel("List elements (10**5)") plt.ylabel("Time (sec)") plt.show()

Python List reverse() In Place

If you call the

Code:

list.reverse()

method on any list object in Python, it reverses the list elements of this particular list object. You say that the reverse method happens in place.

This is a common mistake of many Python beginners. They assume that the

Code:

reverse()

method creates a new list with the elements in reversed order. This is not the case: the

Code:

reverse()

method modifies only the existing list object.

You can see this in the following example:

Code:

>>> lst = [1, 2, 3] >>> lst.reverse() >>> lst [3, 2, 1]

In the example, you only reversed the existing list

Code:

lst

. But you didn’t create a new list!

Python List reverse() None

The return value of the

Code:

list.reverse()

method is

Code:

None

. Why? Because the method reverses the list in place. This means that no new list is created. Instead, the method modifies the old list object.

You’ve seen an example of this in the previous section.

Python List Reverse List Without reverse()

You can also reverse a list without using the reverse() method. Let’s have a look at the following table that shows all reverse() alternatives:

Method |
Description

Code:

lst.reverse()

|
Reverses the order of the elements of list

Code:

lst

in place.

Code:

list(reversed(lst))

|
The built-in

Code:

reversed(lst)

method creates a new list object with reversed list elements.

Code:

lst[::-1]

|
Slicing with negative indexing is the most concise way of reversing the order of a list. It creates a new list object.

Code:

[lst[i] for i in range(len(lst)-1,-1,-1)]

|
Just for fun—one-liner solution to reverse a list using list comprehension and the negative range function.

There is a fifth solution using recursion. But it’s highly inefficient and you shouldn’t use it in practice. If you want to learn about it anyways, read on. But don’t tell me you haven’t been warned!

Python List Reverse Recursive

You can create a recursive function to reverse any list. I’ll give you the code first and explain it later:

Code:

>>> reverse = lambda lst: reverse(lst[1:]) + [lst[0]] if lst else []

Let’s check if it does what it’s supposed to do (reversing the list):

Code:

>>> reverse([1, 2, 3]) [3, 2, 1] >>> reverse(["Ann", 1, 42, 0]) [0, 42, 1, 'Ann'] >>> reverse([]) [] >>> reverse([1]) [1]

Okay, it works!

The recursive one-liner solution uses several Python features you have to understand before you can understand it entirely:

• The lambda function
  Code:

  lambda x: y
  creates an anonymous function with argument
  Code:

  x
  and return value
  Code:

  y
  . You can read the detailed tutorial on the Finxter blog.
  
  
• List concatenation
  Code:

  [a, b, c] + [d]
  creates the new list
  Code:

  [a, b, c, d]
  . You can read the detailed tutorial on the Finxter blog.
  
  
• Slicing
  Code:

  lst[start:stop]
  carves out a subsequence from the list between the
  Code:

  start
  and
  Code:

  stop
  indices. You can read the detailed tutorial on the Finxter blog.
  
  
• The ternary operator
  Code:

  x if y else z
  returns the value
  Code:

  x
  if
  Code:

  y
  is
  Code:

  True
  . In all other cases, it returns the value
  Code:

  z
  . You can read the detailed tutorial on the Finxter blog.
  

Phew! Quite some information to digest! But that’s not all. If you’ve understood all of the above, you also need to understand recursion. That’s too much to teach in a single paragraph so I’d send you over to my blog article about recursion.

I’ll say only that much: to understand recursion, you first need to understand recursion!

Python List Reverse Slice

Slicingis the easiest way to reverse a list.

To reverse the list

Code:

lst

, you simply use slicing operation

Code:

lst[::-1]

with default

Code:

start

and

Code:

stop

indices (not given) and negative step size

Code:

-1

(given).

There’s only one case where you shouldn’t use slicing to reverse the list and this is if you don’t want to create a new list. In this case, stick to the

Code:

lst.reverse()

method which reverses the list in place.

Here’s an example of slicing to reverse a given list:

Code:

>>> friends = ["Ann", "Carsten", "Bob", "Alice"] >>> r_friends = friends[::-1] >>> friends ['Ann', 'Carsten', 'Bob', 'Alice'] >>> r_friends ['Alice', 'Bob', 'Carsten', 'Ann']

You see that the two lists

Code:

friends

and

Code:

r_friends

are independent objects in memory because the slicing operation creates a new list.

Related articles:

• Introduction to Slicing in Python
  

Python List Reverse Copy

There are two ways to copy a list and reverse the order of its elements:

• Use slicing
  Code:

  list[::-1]
  , or
  
  
• Call the
  Code:

  reversed(list)
  method and convert the result to a list using the
  Code:

  list(...)
  constructor.
  

Here are both in action:

Code:

>>> lst_1 = ['Alice', 'Bob', 'Ann'] >>> lst_2 = lst_1[::-1] >>> lst_3 = list(reversed(lst_1)) >>> lst_1 ['Alice', 'Bob', 'Ann'] >>> lst_2 ['Ann', 'Bob', 'Alice'] >>> lst_3 ['Ann', 'Bob', 'Alice']

Python List Partial Reverse

To partially reverse a list

Code:

lst

, use slicing with negative step size:

Code:

lst[start:stop:-1]

. The start and stop values define the part of the list to be reversed and the step size -1 means that you go through the list in reversed order.

Here’s an example of some partial list reversals:

Code:

>>> lst = ['a', 'b', 'c', 'd', 'e'] >>> lst[5:2:-1] ['e', 'd'] >>> lst[:1:-1] ['e', 'd', 'c'] >>> lst[3:2:-1] ['d']

All of those slicing operations reversed a subsequence of the original list. Note that the

Code:

start

index must be larger or equal than the

Code:

stop

index because you traverse the list in negative order (well, if you don’t want to have an empty slice object).

Python List Reverse List Comprehension

You can reverse a list with Python’s powerful list comprehension method. (Although I cannot imagine a scenario where this would actually make sense.)

Related article:

• Introduction to List Comprehension
  

List comprehension is a compact way of creating lists. The simple formula is [ expression + context ].

• Expression: What to do with each list element?
  
  
• Context: What list elements to select? It consists of an arbitrary number of for and if statements.
  

For example, the expression

Code:

[x for x in range(3)]

creates the list

Code:

[0, 1, 2]

.

Here’s how you’d use list comprehension to reverse a list:

Code:

[lst[i] for i in range(len(lst)-1,-1,-1)]

You go over all indices in negative order—starting with the last list index

Code:

len(lst)-1

and ending in the first list index 0. Note that the stop index is not included in the index sequence so I used the value -1 as the stop index for the

Code:

range()

built-in function.

Python List reverse() vs reversed()

What’s the difference between the method

Code:

list.reverse()

and the built-in function

Code:

reversed(list)

?

• Code:

  list.reverse()
  modifies an existing list in place and reverses the order of elements in this list object. No new list object is created.
  
  
• Code:

  reversed(list)
  creates a new iterable object by reversing the order of elements of the original
  Code:

  list
  .
  

So you should use the former if you don’t want to create a new list and the latter if you want to create a new iterable without modifying the existing list.

An example is the following:

Code:

>>> lst_1 = [1, 2, 3] >>> lst_1.reverse() >>> lst_1 [3, 2, 1] >>> reversed(lst_1) <list_reverseiterator object at 0x0000025B58FEC9B0>

The output is not very intuitive but it only means that the

Code:

reversed()

function returns an iterable object.

Python List Deep Reverse

What if you want not only to reverse a list but running a deep reverse where all nested lists are also reversed in a recursive manner?

Here’s how you can do it:

Code:

def deep_reverse(lst): ''' Reverses a nested list in place''' # Reverse top-level list lst.reverse() # Recursively reverse all nested lists for element in lst: if isinstance(element, list): deep_reverse(element) lst = [1, 2, 3, [4, 5, 6]] deep_reverse(lst) print(lst)

This generates the output:

Code:

# OUTPUT: [[6, 5, 4], 3, 2, 1]

Not only the first-level list is reversed but also the second-level list. The code is loosely inspired from this article.

Python List Reverse Enumerate

The

Code:

enumerate(list)

built-in function returns a list of tuples with the first tuple value being the list index and the second tuple value being the list element.

You can reverse the order of enumerated tuples by stacking together the

Code:

enumerate()

function and the

Code:

list.reverse()

method as follows:

Code:

>>> for i, el in enumerate(list(reversed([1, 2, 3]))): print(i, el) 0 3 1 2 2 1

This way, you first reverse the list which creates an iterator. You then transform it into a list. The result can be enumerated.

If you want to reverse the order of the indices as well, simply switch the order of both functions:

Code:

>>> for i, el in reversed(list(enumerate([1, 2, 3]))): print(i, el) 2 3 1 2 0 1

By first enumerating, you calculate the indices based on the original list. Then you reverse them in the outer function.

Python List Reverse Iterator

The

Code:

reversed(list)

method returns an iterator, not a new list. This is different: an iterator is more efficient than a list. You can easily convert the iterator object into a list by using the

Code:

list(...)

built-in function.

Here’s an example:

Code:

>>> reversed([1, 2, 3]) <list_reverseiterator object at 0x0000021735E070B8> >>> for i in reversed([1, 2, 3]): print(i) 3 2 1

The iterator object doesn’t look pretty in the shell but it’s a more efficient way to iterate over a sequence of values than using lists. Why? Because lists need to maintain all values in memory. Iterators don’t.

Python List Reverse Sort

Do you want to sort a list in descending order? Use the

Code:

reverse=True

argument of the

Code:

sorted()

method. Here’s an example:

Code:

>>> sorted([42, 1, 99]) [1, 42, 99] >>> sorted([42, 1, 99], reverse=True) [99, 42, 1]

Python List reverse() Index

Rather than just using positive list indices, you can use reverse indexing in Python lists, too. The negative integer index -1 accesses the last element. The negative integer index -2 accesses the second last element and so on. Here’s an example:

Code:

>>> lst = ["Alice", "Bob", "Ann"] >>> lst[-1] 'Ann' >>> lst[-2] 'Bob' >>> lst[-3] 'Alice'

Python List Reverse range()

Do you want to iterate over a range of integer values in reverse order? Say, you want to iterate over the numbers from 10 to 0 in reverse order: 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0. You can simply achieve this by specifying the

Code:

start

,

Code:

stop

, and

Code:

step

arguments of the

Code:

range(start, stop, step)

method:

Code:

>>> for i in range(10, -1, -1): print(i) 10 9 8 7 6 5 4 3 2 1 0

Note that the

Code:

start

argument is included in the range but the

Code:

stop

argument isn’t.

Python List reverse() Doesn’t Work

What if the

Code:

reverse()

method doesn’t work? Chances are that you assume the

Code:

list.reverse()

method has a return value—that is the reversed list. This is not the case! The

Code:

list.reverse()

method returns

Code:

None

because it reverses the

Code:

list

in place. It doesn’t return a new reversed list.

Here’s an example what you’re probably doing:

Code:

>>> lst = [1, 2, 3] >>> print(lst) [1, 2, 3] >>> print(lst.reverse()) None

If you really want to have a new list with elements in reversed order, use the Python built-in

Code:

reversed(list)

method:

Code:

>>> print(list(reversed([1, 2, 3]))) [3, 2, 1]

The

Code:

reversed()

method reverses the list and returns the reversed list as an iterator object. You need to convert it to a list first before printing it to the shell (and receiving a meaningful output).

Python Reverse List NumPy

To reverse a NumPy array (or even a Python list), you can simply use slicing with negative step size

Code:

a[::-1]

. Here’s an example:

Code:

>>> import numpy as np >>> a = np.array([1, 2, 3]) >>> a[::-1] array([3, 2, 1])

Python List reverse() Thread Safe

Do you have a multiple threads that access your list at the same time? Then you need to be sure that the list operations (such as

Code:

reverse()

) are actually thread safe.

In other words: can you call the

Code:

reverse()

operation in two threads on the same list at the same time? (And can you be sure that the result is meaningful?)

The answer is yes (if you use the cPythonimplementation). The reason is Python’s global interpreter lock that ensures that a thread that’s currently working on it’s code will first finish its current basic Python operation as defined by the cPython implementation. Only if it terminates with this operation will the next thread be able to access the computational resource. This is ensured with a sophisticated locking scheme by the cPython implementation.

The only thing you need to know is that each basic operation in the cPython implementation is atomic. It’s executed wholly and at once before any other thread has the chance to run on the same virtual engine. Therefore, there are no race conditions. An example for such a race condition would be the following: the first thread reads a value from the list, the second threads overwrites the value, and the first thread overwrites the value again invalidating the second thread’s operation.

All cPython operations are thread-safe. But if you combine those operations into higher-level functions, those are not generally thread safe as they consist of many (possibly interleaving) operations.

Where to Go From Here?

The

Code:

list.reverse()

method reverses the order of the list elements.

You’ve learned the ins and outs of this important Python list method.

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