python ctypes pointer operation : can i + or - c pointer type in ctypes?

I have a dll and c code. c code includes many functions not in dll. and I have to make an interface(?) of these functions for python(2.7.x). but the C code uses pointer operation, and I don't know that in Python ctypes module.

see this(not exact code but similar):

void func(unsigned char *arr){
    if( *(arr-1) == *(arr+3) ) {/*...*/}
}

but in python ctypes module, I cannot run a code like this:

t = c_char_p
# .... do sth with variable t?
t -= 2;
print t.value

how can i make it run in python? does any way that i can cast a c_char_p type to integer type or function for modifying the address exist?

I tried c_char_p to c_long casting but it doesn't work...

Thanks in advance!