Hi Pythonians,
To begin with I'd like to apologize that I am not very experienced
Python programmer so please forgive me if the following text does not
make any sense.
I have been missing constants in Python language. There are some
workarounds available, for example the const-module. To me, this looks
quite cumbersome and very unintuitive. For the interpreter, in the
efficiency-wise, I just cannot tell.
For the solution I came up with two new assignment operators to the
language which would create a constant name and constant value: Let's
welcome the new constant assignment operators := and ::=
The := assignment operator says that "declare the name to be constant
and immutable". However, there might be some side-effects and the
constant may not always be constant as you will see below:
a = [1, 2, 3]
b := [4, 5, 6] # assign [4,5,6] to 'b' and declare name 'b' to
be "constant" and "immutable"
c := a # assign 'a' to 'c' and declare that name 'c' is
"constant" and "immutable"
d = c # assign 'c' to 'd' and 'd' inherits the
"immutable"-attribute from 'c', but 'd' can be assigned
later
e = d # assign 'd' to 'e' and 'e' inherits the
"immutable"-attribute from 'd', but 'e' can be assigned
later
a.append( b )
print a # prints [1, 2, 3, 4, 5, 6]
print c # prints [1, 2, 3, 4, 5, 6] because of the side-effect
print d # prints [1, 2, 3, 4, 5, 6] as well
print e # prints [1, 2, 3, 4, 5, 6] no magic here
c = b # 'c' cannot be assigned as 'c' is "constant"
c := b # 'c' cannot be redefined either as 'c' is
"constant"
c.append( 7 ) # cannot be done as 'c' is "immutable"
d.append( 7 ) # cannot be done as 'd' is "immutable" as it was
inherited from 'c'.
e.append( 7 ) # cannot be done as 'e' is "immutable" as it was
inherited from 'd'.
d = [7, 8, 9] # normal variable assignment because 'd' was not
declared to be a "constant"
d.append( 10 ) # now allowed as 'd' is not "immutable" any more
The ::= copy&assign-operator says that "make a copy of the
right-hand-side object and declare the name to be constant and
immutable ". This would give us a true constant object which cannot be
changed. Example follows:
a = [1, 2, 3]
b = [4, 5, 6]
c ::= a # make copy of 'a' and assign that new copy to 'c' and
declare that name 'c' is a "constant" and "immutable"
d = c # assign 'c' to 'd' and 'd' inherits the
"immutable"-attribute from 'c', but 'd' can be assigned
later
e := d # assign 'd' to 'e' and declare that name 'e' is
"constant" and "immutable"
a.append( b )
print a # prints [1, 2, 3, 4, 5, 6]
print c # prints [1, 2, 3] as 'a' and 'c' are two different
objects because of the ::=
print d # prints [1, 2, 3] no magic here
print e # prints [1, 2, 3] no magic here either
c = b # 'c' cannot be assigned as 'c' is "constant"
c := b # 'c' cannot be redefined either as 'c' is
"constant"
c.append( 7 ) # cannot be done as 'c' is "immutable"
d.append( 7 ) # cannot be done as 'd' is "immutable" as it was
inherited from 'c'.
e.append( 7 ) # cannot be done as 'e' is "immutable" as it was
inherited from 'd'.
d = [7, 8, 9] # normal variable assignment because 'd' was not
declared to be a "constant"
d.append( 10 ) # now allowed as 'd' is not "immutable" any more
The := operator would be computionally efficient as it creates only a
reference to an existing object but it may suffer from side-effects.
The ::= is less efficient as it makes a copy on an existing object, but
gives truly constant objects.
Does this make any sense to you, or are there some fatal issues that I
just happened to overlook?
Br,
T.S.
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