How to convert string to int in python

Converting a string to int is a fundamental programming requirement. Most of the times in our code, we get values in string format while many of those values need to be converted to an integer.
Example, a program that adds two numbers and reads those numbers as input from command line, they are read as a string in the program while for performing addition, they should be converted to an integer.
This post will show you the method to convert a string to int in python.


String to int conversion
Python has a built-in int function which takes a string as argument, converts it into an integer and returns the integer value.
Example,

# read from keyboard
in_value = input("Enter a value\n")
print(in_value)
# convert to integer
in_value_int = int(in_value)
print(in_value_int)

Above program reads a value as an input from the keyboard and converts it to an integer. Note that you will not see any difference when a string and int values are printed.
Look at the output of the program below

Enter a value
234
String value: 234
Int value: 234

Both string and int look same right. Now, try to add a value to both string and int and you will see the difference.


Run the below program which adds 2 to both string and integer values.

# read from keyboard
in_value = input("Enter a value\n")
# convert to integer
in_value_int = int(in_value)
# add 2 to integer value
print('Int value:', in_value_int + 2)
# add 2 to string value
print('String value:', in_value + 2)

Above code produces following output

Enter a value
123
Int value: 125
Traceback (most recent call last):
File “test.py”, line 8, in <module>
print(‘String value:’, in_value + 2)
TypeError: must be str, not int

It adds 2 to the integer value and prints it but when adding to the actual input value(which is a string) it gives an error.
This is because you cannot add a number to string directly using + operator.


Converting decimal values
Remember that int function can convert strings which are in integer format. It will fail to parse a string having decimal in it. Thus, if you write int('23.3'), it will throw ValueError: invalid literal for int() with base 10: ‘23.3’
For converting a decimal value to an int, precede the int function with a float function as int(float('23.3')) which will return 23.
float function will first convert the decimal string into a decimal number format. Subsequently, int function will reduce it to an integer by truncating the values after decimal.
Default value of int function
int function takes a string as argument and returns its integer equivalent but if you pass nothing to this function, then it will return 0.
Thus, 0 is the default return value of int function. Example,

default = int()
print("No argument to int function:", default)

Output will be

No argument to int function: 0


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