No there is not. You cannot declare a variable or value as constant in Python. Just don’t change it.
If you are in a class, the equivalent would be:
class Foo(object):
CONST_NAME = “Name”
if not, it is just
CONST_NAME = “Name”
But you might want to have a look at the code snippet Constants in Python by Alex Martelli.
As of Python 3.8, there’s a typing.Final variable annotation that will tell static type checkers (like mypy) that your variable shouldn’t be reassigned. This is the closest equivalent to Java’s final. However, it does not actually prevent reassignment:
from typing import Final
a: Final = 1
# Executes fine, but mypy will report an error if you run mypy on this:
a = 2
There’s no const keyword as in other languages, however it is possible to create a Property that has a “getter function” to read the data, but no “setter function” to re-write the data. This essentially protects the identifier from being changed.
Here is an alternative implementation using class property:
Note that the code is far from easy for a reader wondering about constants. See explanation below
def constant(f):
def fset(self, value):
raise TypeError
def fget(self):
return f()
return property(fget, fset)
class _Const(object):
@constant
def FOO():
return 0xBAADFACE
@constant
def BAR():
return 0xDEADBEEF
CONST = _Const()
print CONST.FOO
##3131964110
CONST.FOO = 0
##Traceback (most recent call last):
## …
## CONST.FOO = 0
##TypeError: None
Code Explanation:
Define a function constant that takes an expression, and uses it to construct a “getter” – a function that solely returns the value of the expression.
The setter function raises a TypeError so it’s read-only
Use the constant function we just created as a decoration to quickly define read-only properties.
And in some other more old-fashioned way:
(The code is quite tricky, more explanations below)
class _Const(object):
@apply
def FOO():
def fset(self, value):
raise TypeError
def fget(self):
return 0xBAADFACE
return property(**locals())
CONST = _Const()
print CONST.FOO
##3131964110
CONST.FOO = 0
##Traceback (most recent call last):
## …
## CONST.FOO = 0
##TypeError: None
Note that the @apply decorator seems to be deprecated.
To define the identifier FOO, firs define two functions (fset, fget – the names are at my choice).
Then use the built-in property function to construct an object that can be “set” or “get”.
Note hat the property function’s first two parameters are named fset and fget.
Use the fact that we chose these very names for our own getter & setter and create a keyword-dictionary using the ** (double asterisk) applied to all the local definitions of that scope to pass parameters to the property function