Callback validation
Validate a callback before passing it to another function.
Python · Function
callable
Tests whether an object appears callable. Classes, functions, and instances with __call__() can return True, but a True result does not guarantee that invocation will succeed.
- Since
- 3.2
How it works
callable() tests whether an object appears usable with call syntax. Functions and classes qualify, as do instances whose class defines __call__; a positive result does not guarantee that invoking the object will succeed.
Syntax
callable(object, /)Parameters
| Name | Type | Description |
|---|---|---|
object | object | The object to test for apparent callability. |
Returns
True if object appears callable; otherwise False.
Key points
- The function returns a Boolean indicating whether its argument appears callable.
- Classes are callable because invoking one creates an instance.
- An instance can be called when its class implements __call__().
- callable() was added in Python 3.2 after being absent in Python 3.0.
Common mistakes
- True is only an indication that the object appears callable; calling it can still raise an exception.
- A False result rules out successfully calling the object.
Example: Checking callable objects
def greet(name):
return f"Hello, {name}"
class Greeter:
def __call__(self, name):
return greet(name)
for item in (greet, Greeter(), 42):
print(callable(item))Checks whether a function, an instance with __call__, or an integer can be invoked.
When to use it
Strategy objects
Distinguish callable strategy objects from ordinary values.
Dynamic invocation
Check dynamically obtained attributes before invoking them.
Related
__call__iter