How can I specify a pointer as a parameter to a function in Python?

Question

I'm trying to define a function (initdeque()) that takes a pointer to an instance of the deque class. So this is what I tried:

from ctypes import *
class deque(Structure):
 pass
deque._fields_ = [("left",POINTER(node)),("right",POINTER(node))]
def initdeque(class deque *p):
 p.left=p.right=None

But this code gives a syntax error:

 def initdeque(class deque *p):
 ^
 SyntaxError: invalid syntax

What is the correct syntax?

• Cédric Julien: what do you want to do with this ? create a deque class implemented in C ?
• David Heffernan: ctypes is most definitely not the way to learn Python

Answer 1

There is no way to specify the type of Python variables, so the declaration of initdequeue should just say:

def initdeque(p):
 p.left = p.right = None

Instead of having a random function doing the initialization, you should consider using an initializer:

class deque(Structure):
 _fields_ = [("left",POINTER(node)), ("right",POINTER(node))]
 def __init__(self):
 self.left = self.right = None

Now, to create a new instance, just write

 p = dequeue()

• kiran: sir, but does this "p" become an instance of the deque class??
• phihag: @kiran No. The type of an object is fixed when it's constructed. Python prefers duck typing over static typing.
• David Heffernan: what's more, fields should be defined in the class rather than outside
• David Heffernan: @kiran I urge you to reconsider learning Python via ctypes. The result will be poor knowledge of both.
• David Heffernan: @kiran I speak from experience. I learnt Python with ctypes and am still trying to recover.

Answer 2

Python (like Matlab) does not have explicit variable type definition. When a variable is created the compiler defines it's type judging from it's content. So (if I am not wrong) you can call the same function with multiple types as parameters and still work. For example

def fun(p):
 return p=p*5

So u can call fun with a string as a parameter and return you a string that contains 5 times the string you sent. Or you can call it with an integer and the function will return you the result of the multiplication