Conventions on creating constants in Python

Question

I am writing an application which needs to find out the schema of a database, across engines. To that end, I am writing a small database adapter using Python. I decided to first write a base class that outlines the functionality I need, and then implement it using classes that inherit from this base. Along the way, I need to implement some constants which need to be accessible across all these classes. Some of these constants need to be combined using C-style bitwise OR.

My question is,

1. what is the standard way of sharing such constants?
2. what is the right way to create constants that can be combined? I am referring to MAP_FIXED | MAP_FILE | MAP_SHARED style code that C allows.

For the former, I came across threads where all the constants were put into a module first. For the latter, I briefly thought of using a dict of booleans. Both of these seemed too unwieldly. I imagine that this is a fairly common requirement, and think some good way must indeed exist!

Answer 1

what is the standard way of sharing such constants?

Throughout the standard library, the most common way is to define constants as module-level variables using UPPER_CASE_WITH_UNDERSCORES names.

what is the right way to create constants that can be combined? I am referring to MAP_FIXED | MAP_FILE | MAP_SHARED style code that C allows.

The same rules as in C apply. You have to make sure that each constant value corresponds to a single, unique bit, i.e. powers of 2 (2, 4, 8, 16, ...).

Most of the time, people use hex numbers for this:

OPTION_A = 0x01 OPTION_B = 0x02 OPTION_C = 0x04 OPTION_D = 0x08 OPTION_E = 0x10 # ... 

Some prefer a more human-readable style, computing the constant values dynamically using shift operators:

OPTION_A = 1 << 0 OPTION_B = 1 << 1 OPTION_C = 1 << 2 # ... 

In Python, you could also use binary notation to make this even more obvious:

OPTION_A = 0b00000001 OPTION_B = 0b00000010 OPTION_C = 0b00000100 OPTION_D = 0b00001000 

But since this notation is lengthy and hard to read, using hex or binary shift notation is probably preferable.