Python automatically assigns a variable the appropriate datatype based on the value it is given. A variable with the value 7 is an integer, 7. is a float, and "7" is a string. Sometimes, we will want to convert variables to different datatypes. For example, if we wanted to print out an integer as part of a string, we would need to convert that integer to a string first. We can do that using str():

age = 13
print "I am " + str(age) + " years old!"

This would print:

>>> "I am 13 years old!"

Similarly, if we have the string "7" and we want to perform arithmetic operations on it, we must convert it to a numeric datatype. We can do this using int():

number1 = "100"
number2 = "10"

string_addition = number1 + number2 
# string_addition now has a value of "10010"

int_addition = int(number1) + int(number2)
# int_addition has a value of 110

If we have a string that is a floating point value, such as "7.5", we can convert it to a numeric datatype using float() instead:

string_num = "7.5"
print float(string_num)

>>> 7.5

Using float() on an integer value, whether string or numeric, will convert the value to a float:

string_int = "10"
numeric_int = 12

print float(string_int)
print float(numeric_int)

>>> 10.0
>>> 12.0

The result of using int() on a floating point value will depend on whether the value is a string or numeric datatype. If you use int() on a floating point string value, it will raise a ValueError:

string_float = "7.5"
print int(string_float)

Traceback (most recent call last):
  File "script.py", line 2, in <module>
    print int(string_num)
ValueError: invalid literal for int() with base 10: '7.5'

Using int() on a floating point numeric value converts the number to an integer by removing the decimal portion and rounding the number closer towards 0:

numeric_float_positive = 7.5
numeric_float_negative = -9.5

print int(numeric_float_positive)
print int(numeric_float_negative)

>>> 7
>>> -9

Note: Use caution when converting a floating point number into an integer, as this may result in the loss of the decimal data.