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Mastering Python: Annotations, Errors and Environment
Mastering Python: Annotations, Errors and Environment
Variable Annotation
Let's create a type annotation for a variable.
To annotate a variable with a type, you need to use a colon (:
) followed by the data type after the variable:
Annotations are meant for the benefit of programmers and do not affect the ability to assign a value of a certain data type to the annotated variable:
number: int = "TheBestString" print(number)
The IDE (Integrated Development Environment) will flag an error if you assign a value of a different type than the one annotated.
Let's take a look at an example in PyCharm:
In the above example, PyCharm alerts us that we are assigning a value of an unexpected data type to the number variable.
Multiple Type Annotations
In Python, you can annotate a variable with multiple types using the |
symbol (available since Python 3.10):
In the above example, PyCharm assigns the float
value (12.24
) to the number variable without any issues.
If you are using Python 3.9 or an earlier version, you can use the imported Union
object from the typing
module to annotate multiple types:
Nested Annotations
A list
data structure can contain values of different types. In cases where you need a list
containing only int
or float
values, nested annotations can be used:
The syntax list[int]
has been available since Python 3.9. If you are using an earlier version of Python, you can import the List
object from the typing
module:
Similarly, these annotations can be used for set data structures.
Annotations can also be created for dict
data structures, where the first position represents the annotation for keys, and the second position represents the annotation for values:
Note
The types inside
dict[]
andDict[]
are separated by commas (,
).
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