Use isinstance(value, str) to check whether a Python value is a string. It returns True for strings, including subclasses of str, and False for other types.
Use isinstance() for the usual runtime check
Pass the value and the str type to Python’s built-in isinstance() function:
value = "hello"
if isinstance(value, str):
print("value is a string")
else:
print("value is not a string")This checks the object’s type relationship at runtime. For example, an integer does not pass the check:
value = 42
print(isinstance(value, str)) # FalseThe Python Software Foundation’s Programming FAQ recommends the built-in isinstance(obj, cls) function for checking whether an object is an instance of a class, including isinstance(obj, str).
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When to use an exact type check
isinstance(value, str) also returns True when value is an instance of a subclass of str. If your code specifically requires the exact built-in str type and should reject subclasses, use:
type(value) is strChoose the check that matches the requirement: use isinstance() for the normal “is this text?” question, and the exact-type check only when subclasses must be excluded.
Do not use str() to test the original type
str(value) converts a value to a string representation; it does not tell you whether the value was already a string. For instance:
value = 42
isinstance(value, str) # False
str(value) # '42'Python’s built-in types documentation describes str as a constructor. Use isinstance() when you need to test the existing value without converting it.
Check bytes separately from text
Python’s str type represents text as immutable sequences of Unicode code points. bytes is a separate type for binary data, so a bytes value does not pass an isinstance(value, str) check:
value = b"hello"
print(isinstance(value, str)) # FalseIf the bytes contain text you need to work with, decode them using the encoding that matches the data:
text = value.decode("utf-8")
print(isinstance(text, str)) # TrueUTF-8 is an example, not a guarantee that every byte sequence uses that encoding. Also, str(value) does not decode bytes: without encoding arguments, it produces a printable representation, such as "b'Zoot!'" for b'Zoot!', as the Python documentation explains.
Runtime checks versus type annotations
A type annotation communicates an expected type to readers and static type checkers; it does not replace a runtime check when your program must inspect an arbitrary value. For a runtime decision, use isinstance(value, str). See Python’s typing documentation for type hints.
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