Python enum auto
Summary: in this tutorial, you’ll learn about the enum auto() function to generate unique values for enumeration members.
Introduction to the enum auto() function
The following example defines an enumeration with three members whose values are 1, 2, and 3:
from enum import Enumclass State(Enum):
PENDING = 1
FULFILLED = 2
REJECTED = 3
Code language: Python (python)
In this example, we manually assign integer values to the members of the enumeration.
To make it more convenient, Python 3.6 introduced the auto() helper class in the enum module, which automatically generates unique values for the enumeration members. For example:
from enum import Enum, autoclass State(Enum):
PENDING = auto()
FULFILLED = auto()
REJECTED = auto()
def __str__(self):
return f'{self.name(self.value)}'
Code language: Python (python)
How it works.
- First, import the
Enumandautoclasses from theenummodule. - Second, call the
auto()to generate a unique value for each member of theStateenumeration.
By default, the auto() class generates a sequence of integer numbers starting from 1.
The following shows the values of the State enumeration’s members:
for state in State:
print(state.name, state.value)Code language: Python (python)
Output:
PENDING 1
FULFILLED 2
REJECTED 3Code language: Python (python)
How enum() auto works
Technically, the auto() calls the _generate_next_value_() method to generate values for the members. Here’s the syntax of the _generate_next_value_() method:
_generate_next_value_(name, start, count, last_values)Code language: Python (python)
The _generate_next_value_() has the following parameters:
nameis the member’s namestartis the starting value of the enum members.countis the number of enum members, including aliases, that have been created.last_valuesis a list of all preceding values used for the enum members.
By default, the _generate_next_value_() generates the next number in a sequence of integers starting from one. However, Python may change this logic in the future.
It’s possible to override the _generate_next_value_() method to add a custom logic that generates unique values. If so, you need to place the _generate_next_value_() method before defining all the members.
The following shows how to override the _generate_next_value_() method to generate values for members by using their names:
from enum import Enum, autoclass State(Enum):
def _generate_next_value_(name, start, count, last_values):
return name.lower()
PENDING = auto()
FULFILLED = auto()
REJECTED = auto()
for state in State:
print(state.name, state.value)
Code language: Python (python)
Output:
PENDING pending
FULFILLED fulfilled
REJECTED rejectedCode language: Python (python)
Summary
- Use enum
auto()class to generate unique values for enumeration members.