Hướng dẫn check if number python

Introduction

In this article, we'll be going through a few examples of how to check if a variable is a number in Python.

Python is dynamically typed. There is no need to declare a variable type, while instantiating it - the interpreter infers the type at runtime:

variable = 4
another_variable = 'hello'

Additionally, a variable can be reassigned to a new type at any given time:

# Assign a numeric value
variable = 4

# Reassign a string value
variable = 'four'

This approach, while having advantages, also introduces us to a few issues. Namely, when we receive a variable, we typically don't know of which type it is. If we're expecting a Number, but receive variable, we'll want to check if it's a number before working with it.

Using the type[] Function

The type[] function in Python returns the type of the argument we pass to it, so it's a handy function for this purpose:

myNumber = 1
print[type[myNumber]]

myFloat = 1.0
print[type[myFloat]]

myString = 's'
print[type[myString]]

This results in:

Thus, a way to check for the type is:

myVariable = input['Enter a number']
if type[myVariable] == int or type[myVariable] == float:
    # Do something
else:
    print['The variable is not a number']

Here, we check if the variable type, entered by the user is an int or a float, proceeding with the program if it is. Otherwise, we notify the user that they've entered a non-Number variable. Please keep in mind that if you're comparing to multiple types, such as int or float, you have to use the type[] function both times.

If we just said if type[var] == int or float, which is seemingly fine, an issue would arise:

myVariable = 'A string'
if type[myVariable] == int or float:
    print['The variable a number']
else:
    print['The variable is not a number']

This, regardless of the input, returns:

The variable is a number

This is because Python checks for truth values of the statements. Variables in Python can be evaluated as True except for False, None, 0 and empty containers like [], {}, set[], [], '' or "".

Hence when we write or float in our if condition, it is equivalent to writing or True which will always evaluate to True.

numbers.Number

A good way to check if a variable is a number is the numbers module. You can check if the variable is an instance the Number class, with the isinstance[] function:

import numbers

variable = 5
print[isinstance[5, numbers.Number]]

This will result in:

True

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Note: This approach can behave unexpectedly with numeric types outside of core Python. Certain frameworks might have non-Number numeric implementation, in which case this approach will falsely return False.

Using a try-except block

Another method to check if a variable is a number is using a try-except block. In the try block, we cast the given variable to an int or float. Successful execution of the try block means that a variable is a number i.e. either int or float:

myVariable = 1
try:
    tmp = int[myVariable]
    print['The variable a number']
except:
    print['The variable is not a number']

This works for both int and float because you can cast an int to float and a float to an int.

If you specifically only want to check if a variable is one of these, you should use the type[] function.

Conclusion

Python is a dynamically typed language, which means that we might receive a data type different than the one we're expecting.

In cases where we'd want to enforce data types, it's worth checking if a variable is of the desired type. In this article, we've covered three ways to check if a variable is a Number in Python.