constants in Python: at the root of the module or in a namespace inside the module?

Question

I’m building a Python module with about a hundred constants.

I would like to avoid naming issues when people import my module so I was wondering what would be the best way to do it.

MY_CONSTANT = 1
MY_SECOND_CONSTANT = 2
...
MY2_CONSTANT = "a"
MY2_SECOND_CONSTANT = "b"
...

Or

class My:
  CONSTANT = 1
  SECOND_CONSTANT = 2
  ...

class My2
  CONSTANT = "a"
  SECOND_CONSTANT = "b"
  ...

Or maybe another of your suggestions?

Coming from Java, I surely prefer the second way, but some might find it overkill…

Asked By: Olivier Grégoire

||

Source

Answer 1

From PEP 8 – Style Guide for Python Code:

Constants

Constants are usually defined on a module level and written in all capital letters with underscores separating words. Examples include MAX_OVERFLOW and TOTAL.

Answered By: Sergey Vedernikov

Answer 2

Every module provides its own namespace, so there’s no need to create another one.

Having module foo.py:

FOO = 1
BAR = 2
SHMOO = 3

you may use it like this:

import foo
foo.BAR

Answered By: Michal Chruszcz

Answer 3

Well, it depends. Usually, constants are defined at module level. But if you have many constants for category_a and category_b, it might even make sense to add a subpackage constants with modules constants.category_a and constants.category_b.

I would refrain from using a class – it could be instantiated, which wouldn’t make sense, and it has no advantage over a module, apart from allowing you to cram more than one into one physical file (which you probably shouldn’t do if there are so many constants). The Java version would probably use a static class, but the Python equivalent is a module.

Name clashes aren’t an issue in Python except when you import * (but you shouldn’t do that anyways: according to the documentation). As long as there are no name clashes inside the module, rest assured that the user will neither pull out all the names from your module into his own, nor import it under a name that clashes with another module.

Answered By: user395760

Answer 4

If you use classes you can forbid overwrite of constants (or forbid even adding constants to that class). Also advantage of using class over files(modules) is when you have many groups you need not to have many files.

So it would look like this:

class MyConstBaseClass: 
    """
    forbids to overwrite existing variables 
    forbids to add new values if "locked" variable exists
    """ 
    def __setattr__(self,name,value):
        if(self.__dict__.has_key("locked")):    
            raise NameError("Class is locked can not add any attributes (%s)"%name)
        if self.__dict__.has_key(name):
            raise NameError("Can't rebind const(%s)"%name)
        self.__dict__[name]=value

class MY_CONST_GRP1(MyConstBaseClass):
    def __init__(self):
        self.const1 = "g1c1"
        self.const2 = "g1c2"
my_const_grp1 = MY_CONST_GRP1()

class MY_CONST_GRP2(MyConstBaseClass):
    def __init__(self):
        self.const1 = "g2c1"
        self.const3 = "g2c3"
        self.locked = 1 # this will create lock for adding constants 
my_const_grp2 = MY_CONST_GRP2()

print my_const_grp1.const1 # prints "g1c1" 
print my_const_grp1.const2 # prints "g1c2"
print my_const_grp2.const1 # prints "g2c1"
print my_const_grp2.const3 # prints "g2c3"
my_const_grp1.new_constant = "new value" #adding constants is allowed to this group
#my_const_grp1.const1 = "new value" #redefine would raise an error
#my_const_grp2.new_constant = "first value" #adding constant is also forbidden
#my_const_grp2.const1 = "new value" #redefine would raise an error

Here is simillar example

Answered By: Vit Bernatik

constants in Python: at the root of the module or in a namespace inside the module?

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Constants