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Boolean & None

In this section, we introduce two more data types, namely boolean (bool) and None (NoneType). Let's start with the latter one, NoneType.

None (NoneType)

NoneType is a special data type in Python that represents the absence of a value.

nothing = None
print(type(nothing))

... which outputs:

>>> Output
<class 'NoneType'>

None can be used as a placeholder for a variable which will be assigned a value later on. Furthermore, if a program was not able to return a value, None can be used as a return value.

Later, None will play a bigger role. For now, we simply keep in mind that None is a thing.

Detour: TypeError

... yet another error.

Often, you’ll want to use a variable’s value within a message. For example, say you want to wish someone a happy birthday. You might write code like this:

age = 23
message = "Happy " + age + "rd Birthday!"
print(message)

... results in.

Traceback (most recent call last):
  File "C:\\IPython\core\interactiveshell.py", line 3577, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-34-80a141e301d6>", line 2, in <module>
    message = "Happy " + age + "rd Birthday!"
              ~~~~~~~~~^~~~~
TypeError: can only concatenate str (not "int") to str

You might expect this code to print the simple birthday greeting, Happy 23rd birthday!. But if you run this code, you’ll see that it generates an error.

This is a TypeError. It means Python encounters an unexpected type in age, as strings were mixed with integers in the expression. We will easily fix the TypeError in the next section.

Casting

When you use integers within strings like this, you need to specify explicitly that you want Python to use the integer as a string of characters. You can do this by wrapping the variable in the str() function, which tells Python to represent non-string values as strings:

age = 23
message = "Happy " + str(age) + "rd Birthday!"
print(message)
>>> Output
Happy 23rd Birthday!

Changing the type of age to string is called casting.

Info

A TypeError can stem from various 'sources'. This is just one common example.

In the following example the integers 3 and 2 were implicitly cast to floating point numbers, to calculate the result, which is a floating point number.

print(type(3 / 2))
>>> Output
<class 'float'>

With the function int() any value can be explicitly cast into an integer, if possible. The value to be cast is passed as the input parameter.

number = 3.0
print(type(number))

# casting
number = int(number)
print(type(number))
>>> Output
<class 'float'>
<class 'int'>

To explicitly cast a value into a float, use the function float().

Casting 
# variables
first = "12"
second = "1.2"
third = 12

For each of the given variables, check whether you can cast them to another type. First, print the type of each variable. Then, use int(), and float() to cast the variables.

Note

Remember the f-string (f"...") from the previous section? Try a slightly modified example from above.

age = 23
message = f"Happy {age}rd Birthday!"
print(message)

You'll notice, that there's no need for any explicit casting of age.

Whenver, you want to include a variable in a string, remember that f-strings might be more convenient. 😉

Booleans

Computers work with binary (e.g. True or False). Such information can be stored in a single bit. A boolean is either True or False. Booleans are often used to keep track of certain conditions, such as whether a game is running or whether a user can edit certain content on a website:

game_active = True
can_edit = False

print(type(True))
>>> Output
<class 'bool'>

Recap

In this section, we have introduced two more data types in Python:

  • None (NoneType)
  • and Booleans (bool)

Now, we have covered all basic types! 🎉 With that knowledge, we can already start to do comparisons and logical operations.