Function in python

Question

I am trying to write a Linear congruential generator in python and I find a little piece of code on Wikipedia but have some difficulty on understanding it. The code is as follows:

def lcg(modulus, a, c, seed=None):
 if seed != None:
 lcg.previous = seed
 random_number = (lcg.previous * a + c) % modulus
 lcg.previous = random_number
 return random_number / modulus
lcg.previous = 2222

My problem is that what is "lcg.previous"? I notice that the function is done, the value of lcg.previous gets updated and stored. Is it declared as a member variable of function lcg() here or actually some kind of default set up for all function in python?

Thanks a lot!

• Alex: Maybe you can give the reference of where you found that code snippet.
• Jan Christoph Terasa: I guess you did not copy all the relevant code, and lcg.previous is set to a starting value directly after the function definition.
• Xuan: Sorry about this, I happen to upload the my edited version. Thanks for editing.
• Daniel Roseman: I wonder if this is very old code. A much better way of achieving this would be with generators, but maybe this was written before they were available.
• Xuan: @DanielRoseman yeah, it is a very old way to generate a uniform random variable

Answer 1

It is a "member variable" of the function, so that each time it is called (except when called with something for seed) the sequence will pick of where it left off.

• Jan Christoph Terasa: +1. As a note, with this you can emulate the behaviour of static function variables common in other languages, like C.
• Xuan: Thanks for your explanation.

Answer 2

Python is recognizing the lcg.previous as a new variable declaration, and will add it as a member to lcg.

• juanpa.arrivillaga: Python doesn't have variable declarations.
• Stephan: It does, it's just implied. the line <code>lcg(modulus, a, c, seed=None):</code> has implied local variable declaration of modulus, a, c, and seed. the line lcg.previous is an implied declaration of a member variable. Python just abstracts away the gritty part of declaring variables under the hood.
• juanpa.arrivillaga: Python, the language, doesn't have a variable declarations. Variables spring into existence when they are assigned to. The language construct of variable declaration simply doesn't exist in Python, as it does in C, C++, Java etc. There is only assignment in Python. You are playing loose with terminology.
• Stephan: Variable declaration is either explicit or implicit. Most compilers interpret an assignment statement as purely an assignment of a value to a memory location. If there is no memory location associated with the reference, you'll at worst get a run time exception, or at best a compiler exception. Python sees that you are trying to use "seed", knows it doesn't have a memory address for it already, so it assigns one for you. Python operates under the assumption that your attempt to use a reference implies that you want the reference to actually exist.

Answer 3

The previous variable is a property on the lcg function. In this example it's being used as a static variable for the lcg function. Since Python doesn't require variables (or object members) to be declared before use you can create them at need. In this case, you've create a member previous of the lcg function object.