How to Convert Strings to Integers in a Python List

You may apply either of the two approaches below in order to convert strings to integers in a Python list:

(1) Using map

my_list = ['value1','value2','value3',...]
my_list = list(map(int, my_list))

(2) Using list comprehension

my_list = ['value1','value2','value3',...]
my_list = [int(i) for i in my_list]

Let’s now review several examples.

Using Map to Convert Strings to Integers in a Python List

Suppose that you have a list that contains 5 values, where each value is stored as a string (by placing each value within quotes):

my_list = ['22','45','75','68','39']
print (my_list)

Here is how the list with the strings would look like:

['22', '45', '75', '68', '39']

You can then use map in order to convert the strings to integers:

my_list = ['22','45','75','68','39']
my_list = list(map(int, my_list))
print (my_list)

Once you run the code, you’ll see the list of integers (without any quotes around the values):

[22, 45, 75, 68, 39]

Optionally, you may perform a further check to verify that the strings were converted to integers:

my_list = ['22','45','75','68','39']
print (my_list)

for i in my_list:
    check = type(i)
    print (i,check)


my_list = list(map(int, my_list))
print (my_list)

for i in my_list:
    check = type(i)
    print (i,check)

The portion in yellow represents the strings prior to the conversion, while the portion in green represents the integers following the conversion:

['22', '45', '75', '68', '39']
22 <class 'str'>
45 <class 'str'>
75 <class 'str'>
68 <class 'str'>
39 <class 'str'>
[22, 45, 75, 68, 39]
22 <class 'int'>
45 <class 'int'>
75 <class 'int'>
68 <class 'int'>
39 <class 'int'>

Using List Comprehension to Convert Strings to Integers in a Python List

Alternatively, you may use the second approach to convert your strings to integers:

my_list = ['value1','value2','value3',...]
my_list = [int(i) for i in my_list]

For our example:

my_list = ['22','45','75','68','39']
my_list = [int(i) for i in my_list]
print (my_list)

The strings will be converted to integers:

[22, 45, 75, 68, 39]

As before, you may apply the syntax below in order to verify that the strings were converted to integers:

my_list = ['22','45','75','68','39']
print (my_list)

for i in my_list:
    check = type(i)
    print (i,check)


my_list = [int(i) for i in my_list]
print (my_list)

for i in my_list:
    check = type(i)
    print (i,check)

You’ll get the same results as the first approach:

['22', '45', '75', '68', '39']
22 <class 'str'>
45 <class 'str'>
75 <class 'str'>
68 <class 'str'>
39 <class 'str'>
[22, 45, 75, 68, 39]
22 <class 'int'>
45 <class 'int'>
75 <class 'int'>
68 <class 'int'>
39 <class 'int'>

Use Case: Why Convert Strings to Integers?

Let’s now review a simple use case, where we’ll have a list with strings. The goal is to deduct 5 from each value within the list:

my_list = ['22','45','75','68','39']

for i in my_list:
    i = i-5

If you try to apply the above code, you’ll get the following error:

TypeError: unsupported operand type(s) for -: ‘str’ and ‘int’

One way to overcome this error is by converting the strings to integers, so that you can perform various arithmetic calculations:

my_list = ['22','45','75','68','39']
my_list = list(map(int, my_list))

new_list = []
for i in my_list:
    i = i-5
    new_list.append(i)

print (new_list)

Once the strings got converted to integers, you’ll be able to successfully deduct 5 from each value within the list:

[17, 40, 70, 63, 34]

Using a list comprehension:

my_list = ['22','45','75','68','39']
new_list = [int(i)-5 for i in my_list]

print (new_list)

And here is the result:

[17, 40, 70, 63, 34]