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Python – Check if Variable is a String

Since Python does not support static type checking (i.e type checking at compile type), if you ever want to check if a Python variable or object is a String or not; we need to use certain methods.

Let us understand some of the ways of checking for a string type object.


1. Using isinstance() method

The isinstance(object, type) method checks if object belongs to type, and returns True if that condition holds, and False otherwise.

Common types are: int, str, list, object, etc.

Since we want to check for str type, we will use isinstance(object, str) and check if it is True.

Let us understand this through an example.

a = 123

b = 'Hello'

print('Is a an instance of str?', isinstance(a, str))
print('Is b an instance of str?', isinstance(b, str))

Output

Is a an instance of str? False
Is b an instance of str? True

2. Using type(object) method

This is similar to the isinstance() method, but this explicitly returns the type of the object.

Let us use this to check if the returned type is str or not.

Example:

a = 123

b = 'Hello'

print('type(a) =', type(a))
print('type(b) =', type(b))

print('Is a of type string?', type(a) == str)
print('Is b of type string?', type(b) == str)

Output

type(a) = <class 'int'>
type(b) = <class 'str'>
Is a of type string? False
Is b of type string? True

Check if Function parameter is a String

We can apply any one of the above methods to introduce a ‘check’ condition in any function, which allows us to execute the main body of the function only if the input is a string.

Let us understand this using a simple example.

a = 123

b = 'Hello'

def capitalize_name(inp):
    # We can also use "if isinstance(inp, str)"
    if type(inp) != str:
        print('Input must be a string')
    else:
        print(inp.upper())


capitalize_name(a)
capitalize_name(b)

Output

Input must be a string
HELLO

Our function now explicitly checks if the parameter is a string, before proceeding to the main body. These type checks can potentially avoid unnecessary run-time errors due to Python’s dynamic type checking.

We are not throwing any errors from our function. But, in real life programming, most of these function parameter type validations throw TypeError exception.


Conclusion

In this article, we learned about how we could use Python’s isinstance(), type() methods and check if the input is a string or not. We also applied this to a function to only accept a parameter if it is a string.

Now, there are potentially much lesser errors that could be caused during run-time, and this format is very essential for a good Python program. Hope you found this post useful!

References

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