Let's look at different methods for adding elements to the beginning of a list and string: concatenation, insert, append, extend, rjust, and f-strings.
How to add an item to the top of a Python list: 4 methods
Lists in Python are mutable and ordered data structures designed to store a group of values. If you need to insert an element in the first position of an existing list during the program execution, you can use one of the methods described below.
Method 1: insert
This method is implemented using the built-in insert() method of lists. This method works with any type of data and allows you to insert the desired element at any position in the existing list, including the first one.
The insert() method takes two parameters – the index (position) at which the element is to be inserted, and the element itself. Counting positions in Python starts from zero – Accordingly, to insert an element at the beginning of the list , you need to specify 0, and not, as the first parameter 1.
Let's take an example. Suppose we have a list [1, 2, 3] where we want to insert the number 5 at the beginning. Use the insert() method:
arr = [1, 2, 3]
arr.insert(0, 5)
print(arr)
Result:
[5, 1, 2, 3]
Method 2: Concatenation
Using concatenation (addition), you can also insert the desired element at the beginning of the list. However, this requires that the element is presented as a list:
arr = [1, 2, 3]
print([5] + arr)
The result will be the same as in the first method:
[5, 1, 2, 3]
Method 3: The append method
By default, this method expands an existing list by adding an element to the end. But nothing prevents you from expanding the list consisting of a single element:
arr = [1, 2, 3]
num = [5]
num.append(arr)
print(arr)
The result when using append() is different – you will get a nested list:
[5, [1, 2, 3]]
The desired element, however, gets an index 0, so the goal is completed. In addition, when solving some tasks, you may need to create a nested list, so this method has the right to live.
Method 4: extend method
The extend() method is similar to append () – with the difference that it preserves the" one-dimensionality " of the list:
arr = [1, 2, 3]
num = [5]
num.extend(arr)
print(num)
The result is a regular, non-nested list:
[5, 1, 2, 3]
Note that one-dimensionality is preserved even if the element to be placed at the beginning of the list is itself a list consisting of several elements:
arr = [1, 2, 3]
num = [5, 6, 7]
num.extend(arr)
print(num)
Result:
[5, 6, 7, 1, 2, 3]
4 ways to add an element to the beginning of a string in Python
Strings in Python are an immutable data type str, and represent sequences of different characters. Since the lines do not change, you can only add elements to the beginning of the sequence by creating a new line.
Method 1: Concatenation
Strings in Python can be concatenated (the result will be a new line). Let's look at the example of inserting a sign +at the beginning of a string containing an abstract mobile number:
el = '+'
num = '123456'
print(el + num)
The operation results in a new line:
+123456
Method 2: Using the f-string
You can also use the f-string to insert an element at the beginning of a line:
el = '+'
num = '1337'
print(f'{el}{num}')
The result will be similar to the first method:
+1337
Method 3: Converting to a list
If you need to insert an element at a specific position in the string, including at the beginning, you can use the join() method to convert the string to a list and merge the list into a string.:
el = '+'
num = list('9876')
(num).insert(0, el)
print(''.join(num))
The result will be the same:
+9876
Method 4: Using rjust
The rjust() method is used to align the line to the right edge. As parameters, it accepts the length of a new line and the character that will be used to fill in empty positions. By default, a space is used as a placeholder, but nothing prevents you from selecting a sign +:
num = "5678"
print(num.rjust(12, '+'))
The result is the desired string with the required character inserted at the beginning:
+5678
Let's sum up the results
We've covered eight simple and practical ways to add the desired element to the beginning of a Python list and string. Do you know any other interesting ways to insert elements? Share it with us in the comments.
Top comments (2)
Fifth possible method:
arr = [1, 2, 3]
arr[0:0] = [0]
especially handy if you want to insert a list in the middle of another one.
Pretty good! Thank you