To customize the behavior of the negation operator -x, override the __neg__(self) dunder method in your class definition. Python internally calls x.__neg__() to calculate the inverse (negation) of an object, i.e., -x. If the __neg__() method is not defined, Python will raise a TypeError.
Syntax
__neg__(self)To use the negation operator -x on a custom object x, define the __neg__() “dunder” magic method that takes one argument: a reference to itself, called self. You can then use attributes of the custom objects to determine the object’s negation. It can return any object but the convention is to return an object of the same type so that -(-x) == x.
Let’s have a look at an example next.
Example
In the following code, you create a Person object alice with a string name attribute set to 'alice'. The negation is simply the name string in reverse order by using slicing with negative step size self.name[::-1].
class Person:
def __init__(self, name):
self.name = name
def __neg__(self):
return self.name[::-1]
alice = Person('alice')
print(-alice)
# ecilaThe negated object -alice returns the attribute 'alice' in reverse order as defined in the __neg__() method. However, this is not a clean way of using the __neg__ dunder method. Instead, you should always return the same object (per convention) so that the inversion criterion is fulfilled, i.e., -(-x) == x assuming the equality dunder method is properly defined.
You can see a proper use in the following example:
class Person:
def __init__(self, name):
self.name = name
def __neg__(self):
return Person(self.name[::-1])
def __eq__(self, other):
return self.name == other.name
alice = Person('alice')
print(-(-alice) == alice)
# True
[Fixed] TypeError: bad operand type for unary –
If you attempt to use the unary operator -x on an object x that doesn’t define the __neg__ dunder method, Python raises a TypeError: bad operand type for unary.
class Person:
def __init__(self, name):
self.name = name
alice = Person('alice')
print(-alice)
Here’s the error message of this code snippet:
Traceback (most recent call last):
File "C:\Users\...\code.py", line 8, in <module>
print(-alice)
TypeError: bad operand type for unary -: 'Person'To fix this error, simply define the unary __neg__() dunder method in the class definition of the object on which you attempt to call the negation operator -.
This is exemplified here:
class Person:
def __init__(self, name):
self.name = name
def __neg__(self):
return Person(self.name[::-1])
alice = Person('alice')
neg_alice = -alice
print(neg_alice.name)
# ecila
Where to Go From Here?
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