Use isinstance(value, str) to check whether a value is a string at runtime. It returns True for strings, including instances of str subclasses, and False for other types.
Use isinstance() for the usual runtime check
Pass the variable and str 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")The test checks the value’s type relationship at runtime. Python’s Programming FAQ specifically recommends using isinstance(obj, cls) for this purpose.
When to use an exact-type check instead
isinstance(value, str) also returns True when value is an instance of a subclass of str. If your requirement is specifically that the value’s type be the built-in str type—not a subclass—use:
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- Read Before You Buy — No Video Output: These adapters support charging and USB 2.0 data transfer, but cannot transmit video signals. Except for standard USB webcams (which use USB data only), they are not compatible with HDMI/DisplayPort cables, video-capable USB-C hubs, or docking stations with video output.
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if type(value) is str:
print("value has the exact built-in str type")For ordinary string checks, prefer isinstance(); reserve type(value) is str for cases where excluding subclasses is intentional.
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 original value was already a string. For example, str(42) returns '42', but 42 is an integer:
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- 5-in-1 USB-C Hub: Experience comprehensive connectivity featuring a Power Delivery input, two USB-A 2.0 ports, a USB-A 3.0 port, and an HDMI port. (Note: The USB-C power delivery input port is only for connecting an external wall charger to power your laptop and cannot power peripheral devices.)
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value = 42
isinstance(value, str) # False
str(value) # '42'Use isinstance() when the question is about the type of the existing value. Use str() only when you actually want a string representation.
How strings differ from bytes
In Python, str represents text as an immutable sequence of Unicode code points. bytes is a separate type for binary data, so a bytes value does not pass an isinstance(value, str) check. The built-in types documentation describes the string type and its behavior.
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value = b"hello"
isinstance(value, str) # False
text = value.decode("utf-8")
isinstance(text, str) # TrueDecode bytes only when you intend to treat them as text, and use the encoding that matches the data. UTF-8 is common, but it is not guaranteed for every byte sequence. Calling str() on bytes without encoding arguments does not decode them: for example, str(b'Zoot!') produces the printable representation "b'Zoot!'". See Python’s documentation for str.
Runtime checks versus type annotations
A type annotation can communicate that a function expects a string and help static type checkers analyze code. It does not, by itself, check an arbitrary value at runtime. When your program needs to make a runtime decision about a value, use isinstance(value, str). Python’s typing documentation explains the role of annotations and static typing.
Quick Recap
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- 5-in-1 Connectivity: Equipped with a 4K HDMI port, a 5 Gbps USB-C data port, two 5 Gbps USB-A ports, and a USB C 100W PD-IN port. Note: The USB C 100W PD-IN port supports only charging and does not support data transfer devices such as headphones or speakers.
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- Please Note: To reach 10Gbps speed, keep the cable under 3.3 ft. For USB A Male to USB C adapters, try flipping the USB C connector. USB C Male to USB A adapters support bidirectional 10Gbps transfer within 3.3 ft
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