__dir__() magic method implements the functionality of the
dir() built-in function. Semantically,
dir() returns all (function, object, or variable) names in a given scope. However, the magic method
__dir__() converts any return value to a sorted list.
The following code defines a custom class
My_Class and overrides the
__dir__() magic method to return a dummy list
[1, 2, 3] when calling
dir(x) an object
x of type
class My_Class: def __dir__(self): return [1, 2, 3] x = My_Class() print(dir(x)) # [1, 2, 3]
Before we dive into some more advanced examples, let’s have a look at the purpose of the
dir() function first!
If used without argument, Python’s built-in
dir() function returns the function and variable names defined in the local scope—the namespace of your current module.
If used with an object argument,
dir(object) returns a list of attribute and method names defined in the object’s scope.
Python __dir__() Sorted Output
The output of any
dir() function call must be a sorted list (source). Even if you override it by defining your custom magic method
__dir__(), Python will attempt to convert your custom output to a sorted list.
You can see this in the following example where your custom
__dir__() returns an unsorted list—and Python brings it back to order!
class My_Class: def __dir__(self): return ['Bob', 'Alice', 'Carl'] x = My_Class() print(dir(x)) # ['Alice', 'Bob', 'Carl']
The original list
['Bob', 'Alice', 'Carl'] was automatically and implicitly converted to the sorted list
['Alice', 'Bob', 'Carl'].
Python __dir__() List Elements of Different Type
__dir__() returns a list with elements of different types—that is—types with incompatible comparison operators (
>=), Python will raise a
TypeError: '<' not supported between instances of 'x' and 'y'.
class My_Class: def __dir__(self): return ['Bob', 'Alice', 42] x = My_Class() print(dir(x))
Traceback (most recent call last): File "C:\Users\...\code.py", line 7, in <module> print(dir(x)) TypeError: '<' not supported between instances of 'int' and 'str'
To fix the error, make sure that all elements in the list have compatible comparison operators.
You can accomplish this by defining the magic methods
__le__ so that comparison of the diverse types is possible.
Alternatively, you can make sure that all elements of the list returned by
__dir__ are of the same type.
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
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