Int

In Python we use int() to convert values. The int keyword is not used to declare values like in other languages. Instead it converts strings and floats.

For floats, int will always truncate the number (remove the part after the decimal place). So 1.9 will become 1.1. For strings, only valid strings can be converted without an error.

Numbers example

Here we use int() on some floating-point numbers. We see that the part after the decimal place is removed—the fractional value is discarded.

# Use int to convert floats to integers.
# ... The value is always rounded down (truncated).
value = 1.5
result = int(value)
print(value, "INT =", result)

value = 1.9
result = int(value)
print(value, "INT =", result)

value = -1.9
result = int(value)
print(value, "INT =", result)
1.5 INT = 1
1.9 INT = 1
-1.9 INT = -1

Parse string

Now let's use int() on a string value. If the string contains only digits like "123" then we can transform the string into a number. We test it in an if-statement.

# Convert a string containing an integer to an int.
data = "123"
result = int(data)
print(data, "INT =", result)

# The result is an integer value.
if result == 123:
    print(True)
123 INT = 123
True

Int error

With int we must be careful not to try to parse a string that does not contain an integer value. A ValueError will occur, and this is disruptive.

# This fails as the string is not in a valid format.
data = "cat"
result = int(data)
Traceback (most recent call last):
  File "C:\programs\file.py", line 6, in <module>
    result = int(data)
ValueError: invalid literal for int() with base 10: 'cat'

Isdigit

With isdigit() we can test a string for an integer value before converting it to an int with int(). This prevents the ValueError on invalid strings.

values = ["123", "cat", "bird", "12.34", "0"]

# Loop over strings.
for v in values:
    # See if string is all digits.
    if v.isdigit():
        # Parse string with int.
        result = int(v)
        print(v, "INT =", result)
    else:
        # String will fail parsing.
        print(v, "ISDIGIT =", False)
123 INT = 123
cat ISDIGIT = False
bird ISDIGIT = False
12.34 ISDIGIT = False
0 INT = 0

Int is an important built-in function in Python. It is not used to specify an integer like 5. But it can convert a string like "5" to 5, or a float like 5.3 to an integer.