Implement class method based on instance variable values (OOP)

I am new to OOP. I want to create objects that have a method which depends on the instance variables value.

class cases:
    def __init__(self, a):
        self.a = a  
  
    def f1(self, x):
        return x

    def f2(self, x):
        return 100*x
        
    if self.a < 3:
        self.x = f1
    else:
        self.x = f2

If I create the following instances c1 and c2 by the class cases:

c1 = cases(2)
c2 = cases(5)

and execute the method x() I expect that:

c1.x(10)  # --> 10
c2.x(10)  # --> 1000

But right now the error is:

NameError: name 'self' is not defined

If I change self.a to just a in the class definition, the error is:

NameError: name 'a' is not defined

So how can I access the variable instance for a case discrimination?

UPDATE
By following the instructions of @deceze (2nd try). Code becomes the followed:

def f1(x):
    return x

def f2(x):
    return 100*x

class case:
    def __init__(self, a):
        self.a = a
       
    def x(self, x):
        if self.a < 3:
            return f1(x)
        else:
            return f2(x)
          
# make instances with different cases    
c1 = case(2)
c2 = case(4)

# show results
print(c1.x(10))
print(c2.x(10))

Result is as desired.

You are mixing up instances and parameters.

def f1(x):
    return x

def f2(x):
    return 100*x



class case:
    def __init__(self, a):
        self.a = a
   
    def x(self, a):
        if self.a < 3:
            return f1(a)
        else:
            return f2(a)
      
# make instances with different cases    
c1 = case(2)
c2 = case(4)

# show results
print(c1.x(10))
print(c2.x(10))

Output as expected.