How do you convert a list of strings to a list of integers in Python? And can you do it with one line of code?
To convert a list of strings to a list of integers use the built-in
map()
function if you know the contents of the original list will
all
convert to integers, otherwise use a
lambda
function in your
map()
, or use a list comprehension with conditions.
Let’s look at how you can change your original list using these approaches below.
Modify List Using
map()
The built-in
map(fn, iter)
takes 2 parameters: the first is the function (
fn
) to modify each element in the second parameter’s iterable (
iter
). The
fn
needs just one parameter as the
map()
function inserts each element from the iterable into the function.
If you have a list of strings that could
all
neatly change to integers you could solve this problem by inserting the
int()
built-in method (which only takes one parameter) and using this as the first parameter to the
map()
function.
Here’s how this would look:
>>> my_list = ['1', '2', '3']
>>> map(int, my_list)
<map object at 0x11beae770>
As you can see from the above result from my Python REPL the output is a
map object
.
To inspect the contents of the
map object
you could use the
print()
function with an
asterisk operator
on the
map
function, but as this would make it hard to determine if there was an
actual change
to the elements I’m going to wrap the result in the
list()
function.
Here’s what you would see inspecting the
map object
:
>>> print(*map(int, my_list))
1 2 3
>>> list(map(int, my_list))
[1, 2, 3]
>>> print(my_list)
['1', '2', '3']
From the above code, you can see that by using the
asterisk operator
it’s difficult to determine if the values have changed. However, using the
list()
built-in function clearly shows the values in the new list are integers not strings – and you can see the difference between a list of strings (the original list) and the new list with how they are both output to the REPL.
What’s more notice how the original source list is
not modified
. Therefore the
map()
function
does not mutate
the iterable being passed into its
second parameter
.
Therefore, to change a list of strings to a list of integers you could use the following one-liner:
list(map(int, string_list))
Where
string_list
is the source variable that contains the list of strings.
But what if the original list of strings contains strings that cannot be converted to an integer?
Suppose you have the following list with an element within the list that cannot easily convert to an integer using the
int()
function:
>>> my_list = ['1', '2', 'e']
>>> list(map(int, my_list))
Traceback (most recent call last):
File "<console>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'e'
As you can see trying to force conversion on an element that will not change to an integer using the
int()
method will produce a
ValueError
.
This is where if it’s a possibility your source list may contain elements that will not readily convert to integers that you will need to apply filtering .
Using
lambda
With
if
As Filter In
map
Function
One means of being able to provide a filter on values being iterated through using the
map
function is to replace
fn
parameter (the first parameter in the
map
function) which I’ve currently had as the
int()
function is to change it with a custom
lambda
function.
As the
map()
function passes each iteration to the
fn
function in the first parameter the
lambda
function just needs one parameter, like so:
lambda x: rest of function goes here
It’s then a matter of working on the right side of the
lambda
call to produce what you want to return.
Our case here is to determine if the passed in value (represented by the parameter
x
for each value from the list of strings) will convert to an integer, if not then you would likely discard it and move on.
How To Convert String To Integer Without Using
try-catch
One method of checking
if a string will change to an integer
without using a
try
statement is to replace all the numbers in the string with an
empty string
and to see if there is anything left over.
The principle looks something like this:
>>> import re
>>> a_string = '123'
>>> len(re.sub(r'\d+', '', a_string))
0
>>> b_string = '123A123'
>>> len(re.sub(r'\d+', '', b_string))
1
As you can see from the two examples above, after importing the regex library using the
import re
statement I created a string labelled
a_string
containing all integers. Using the
re.sub()
(regex substitute) method I inserted into its three parameters the regex pattern to find all digits (
r'\d+'
), first, the
empty string
''
to substitute for each digit found, second, and finally the string to perform the operation on.
By wrapping the
re.sub()
with
len()
I can then determine the length of the string remaining
after
the substitutions have happened. In my first example above the final string had 0 length as all characters in the string were digits, whereas the second example had a length of 1 as one character in the string was
not a digit
.
Using this same approach in the
lambda
function would look something like this:
lambda x: int(x) if len(x) > 0 and len(re.sub(r'\d+', '', x)) == 0 else None
In this
lambda
function above I’ve assumed import of the regex library. If you want to exclude importing a library you could make use of the built-in string method
str.replace()
which does the same thing, however, each digit would need to be chained together which makes the expression
very long
.
Here’s what the above
lambda
expression would look like without an
import re
statement and using the
str.replace()
approach:
lambda x: int(x) if len(x) > 0 and len(x.replace('0', '').replace('1', '').replace('2', '').replace('3', '').replace('4', '').replace('5', '').replace('6', '').replace('7', '').replace('8', '').replace('9', '') == 0 else None
As you can see when comparing both approaches the importing of the Regex library is a lot smaller, simpler and therefore less prone to mistakes!
Besides the substitution of the digit strings to empty strings I’ve also added a preliminary check to make sure a value is being passed into the
lambda
function by writing the code
len(x) > 0
. This would prevent any false positives going through.
>>> a_var = ''
>>> int(a_var) if len(re.sub(r'\d+', '', a_var)) == 0 else None
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: ''
As you can see the original variable
a_var
holding an empty string can slip through the cracks if there’s no check initially on the length. Therefore, the
len(x) > 0
is needed to prevent this from happening.
What If Integer String Contains Thousands Separators (Like Comma Or Period)?
An integer string can still be valid but it may contain other characters for separation of thousands places such as
1,000,000
to denote one million. If this is needed for your use case then you can add in the substitution the commas or periods needed.
Here’s how this would look for each approach:
lambda x: int(x.replace(',', '')) if len(x) > 0 and len(re.sub(r'\d+|,', '', x)) == 0 else None
lambda x: int(x.replace(',', '')) if len(x) > 0 and len(x.replace('0', '').replace('1', '').replace('2', '').replace('3', '').replace('4', '').replace('5', '').replace('6', '').replace('7', '').replace('8', '').replace('9', '').replace(',', '') == 0 else None
Testing the result of our code in the Python REPL would look something like this:
>>> import re
>>> a_list = ['1,234', '5678', 'ABCD']
>>> list(map(lambda x: int(x.replace(',', '')) if len(x) > 0 and len(re.sub(r'\d+|,', '', x)) == 0 else None))
[1234, 5678, None]The outcome is achieved using the code above to help remove any thousand separators, such as a comma, and all in one line of code!
But notice the result produces a list the same size as the original with
None
for any elements that couldn’t readily convert to an integer.
You could change the
lambda
function to report the elements that couldn’t be changed by swapping the
None
in the
if-else
statement with
x
, like this:
>>> import re
>>> a_list = ['1,234', '5678', 'ABCD']
>>> list(map(lambda x: int(x.replace(',', '')) if len(x) > 0 and len(re.sub(r'\d+|,', '', x)) == 0 else x))
[1234, 5678, 'ABCD']Reporting the original elements back into the list could be helpful if more work is needed on converting the strings.
If the new list is to remove any elements that will not be converted to integers you can use another approach using list comprehensions.
Filter And Remove Elements Without Using
map()
To filter elements from an original list based on certain conditions you can use the handy list comprehension.
With the current working examples here’s how this would work:
>>> a_list = ['1,234', '5678', 'ABCD']
>>> [int(x.replace(',', '')) for x in a_list if len(x) > 0 and len(re.sub(r'\d+|,', '', x)) == 0]
[1234, 5678]
The biggest difference with the list comprehension compared to the
map()
function is handling the
else
cases. In the list comprehension above the
if
condition filters each element according to the condition and if it does not meet this criteria the element is not passed through to the front of the
for
statement.
Summary
The list comprehension provides a simple one-liner piece of code that can easily convert a list of strings to integers and remove any elements within the list that cannot be easily converted to integers.
If it’s important to place something into the new integer list or to show elements from the original list that cannot be changed you could report back the original element or a default value such as
None
or
0
in replacement.
