Type Casting and None

15 min · Beginner

Data from users or APIs often arrives as text. Type casting converts data into the correct type for calculations.

Converting strings to numbers

Type casting is converting a value from one type to another, for example text "42" to the number 42 with int(). The original object is not changed; a new one is returned.

Use int() to convert numeric text into an integer, and float() to convert into a decimal number.

Python

text_count = "42"count = int(text_count)print(count + 8)

Example 1 · int() cuts the decimal part

Python

print(int(3.9))print(int(-3.9))

Output

3
-3

What this code does

int() drops the decimal part; it does not round.
For negative numbers it moves toward zero, so -3.9 becomes -3.

The console prints 50. Without int(), adding '42' + 8 raises a TypeError because Python does not perform implicit string-to-number coercion.

Handling conversion errors

ValueError is the error Python raises when a value has the right type but cannot be used, such as int("abc"). The program stops unless the error is handled.

Passing non-numeric text to int() raises a ValueError at runtime.

Python

price_text = "99.50"price = float(price_text)print(price)

Notice that int("99.50") fails with a ValueError because the string contains a decimal point. Convert to float first or parse cleanly.

Converting numbers to strings

Use str() to turn any integer, float or boolean into a printable string representation.

Python

status_code = 200message = "HTTP " + str(status_code)print(message)

Example 2 · Text from a form becomes a total

Python

1raw_input = "250"2quantity = int(raw_input)3unit_price = 14.54total = quantity * unit_price5
6print(total)7print(type(total))8print(str(total) + " USD")

Output

3625.0
<class 'float'>
3625.0 USD

What this code does

raw_input is converted from string to integer.
total multiplies integer by float, resulting in a float (3625.0).
str(total) converts the number back to text for string concatenation.

The console displays HTTP 200. Joining strings with the + operator requires all parts to be strings.

The None singleton

None is the single value that means no value. Its type is NoneType. Use it for "nothing here yet", and test for it with the is operator.

Python uses None to represent the absence of a value or a null reference. Its type is NoneType.

Python

cached_result = Noneprint(cached_result)print(cached_result is None)

Example 3 · A value that is not there yet

Python

middle_name = Noneprint(middle_name is None)print(type(middle_name))

Output

True
<class 'NoneType'>

What this code does

middle_name is bound to None, the "no value" object.
is None is the correct way to test for it.
Its type is NoneType.

Always check for None using the identity operator is None, not equality == None.

Next step

Check your understanding

9 quick questions

Type Casting and None

15 min · Beginner

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Data from users or APIs often arrives as text. Type casting converts data into the correct type for calculations.

Converting strings to numbers

Type casting is converting a value from one type to another, for example text "42" to the number 42 with int(). The original object is not changed; a new one is returned.

Use int() to convert numeric text into an integer, and float() to convert into a decimal number.

Python

text_count = "42"count = int(text_count)print(count + 8)

Example 1 · int() cuts the decimal part

Python

print(int(3.9))print(int(-3.9))

Output

3
-3

What this code does

int() drops the decimal part; it does not round.
For negative numbers it moves toward zero, so -3.9 becomes -3.

The console prints 50. Without int(), adding '42' + 8 raises a TypeError because Python does not perform implicit string-to-number coercion.

Handling conversion errors

ValueError is the error Python raises when a value has the right type but cannot be used, such as int("abc"). The program stops unless the error is handled.

Passing non-numeric text to int() raises a ValueError at runtime.

Python

price_text = "99.50"price = float(price_text)print(price)

Notice that int("99.50") fails with a ValueError because the string contains a decimal point. Convert to float first or parse cleanly.

Converting numbers to strings

Use str() to turn any integer, float or boolean into a printable string representation.

Python

status_code = 200message = "HTTP " + str(status_code)print(message)

Example 2 · Text from a form becomes a total

Python

1raw_input = "250"2quantity = int(raw_input)3unit_price = 14.54total = quantity * unit_price5
6print(total)7print(type(total))8print(str(total) + " USD")

Output

3625.0
<class 'float'>
3625.0 USD

What this code does

raw_input is converted from string to integer.
total multiplies integer by float, resulting in a float (3625.0).
str(total) converts the number back to text for string concatenation.

The console displays HTTP 200. Joining strings with the + operator requires all parts to be strings.

The None singleton

None is the single value that means no value. Its type is NoneType. Use it for "nothing here yet", and test for it with the is operator.

Python uses None to represent the absence of a value or a null reference. Its type is NoneType.

Python

cached_result = Noneprint(cached_result)print(cached_result is None)

Example 3 · A value that is not there yet

Python

middle_name = Noneprint(middle_name is None)print(type(middle_name))

Output

True
<class 'NoneType'>

What this code does

middle_name is bound to None, the "no value" object.
is None is the correct way to test for it.
Its type is NoneType.

Always check for None using the identity operator is None, not equality == None.

Next step

Check your understanding

9 quick questions