Passing a class instance method to another function in Python (2.7)

Question

sorry if the title does not make sense, I am relatively new to this. This is my code:

class MeanFlow:
    def __init__(self, V0=1):
        self.V0 = V0
    def LHS(self, t, y):
        return y[0]*self.V0


def velocity_field(w,f):
    z = 0 # dummy 
    u = f(z,w).real
    v = -1*f(z,w).imag 
    return u, v

w0 = 1
mean = MeanFlow()
dwdz = mean.LHS
print(velocity_field(w0, dwdz))

But I get the error TypeError: 'int' object has no attribute '__getitem__'

My question is how do I pass this function which is a method of my class instance into another function. If I define the function outside the class and pass it to another function this works but is not what I want. Thanks!

Edit: The typo return = y[0]*self.V0 has been corrected.

The code you've shown doesn't demonstrate the error you've posted. Instead, it fails to compile due to the line return = y[0]*self.V0. — user2357112
– user2357112, Commented Apr 2, 2014 at 10:50

rickcnagy · Accepted Answer · 2014-04-02 11:12:05Z

3

What's generating TypeError: 'int' object has no attribute '__getitem__' is this:

y[0]

This is because at this point, y's value is 1, an integer, and y[0] is acting as if y is a list or string (__getitem__ is the method called to get items in lists). If y were a list (e.g. y = [1]), it'd work fine.

If you remove the [0], you're in business:

class MeanFlow:
    def __init__(self, V0=1):
        self.V0 = V0
    def LHS(self, t, y):
        return y*self.V0


def velocity_field(w,f):
    z = 0 # dummy 
    u = f(z,w).real
    v = -1*f(z,w).imag 
    return u, v

w0 = 1
mean = MeanFlow()
dwdz = mean.LHS
print(velocity_field(w0, dwdz))

edited Apr 2, 2014 at 11:12

answered Apr 2, 2014 at 10:46

rickcnagy

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5 Comments

Dipole Over a year ago

Sorry, the assignment to return was completely wrong of me, I didn't spot that. And thanks for the kind response :)

rickcnagy Over a year ago

No problem! Mistakes like that are very hard to spot, especially when you've written the code yourself. Often my solution is to take a step back and imagine what the values of the variable should be in theory at each point along the way, then use print to find what they actually are.

Dipole Over a year ago

As a followup I am wondering why it is not possible to then define w0 = [1] and use the method involving y[0]?

rickcnagy Over a year ago

Sure - that'll work if we go back to y[0], so return y[0]*self.V0. However, if you set w0=[1] and then use return y*self.V0, you'll end up doing this: [1] * 1, which returns [1] (see this SO post), and .real is a property of numbers, not lists

Dipole Over a year ago

@l4mpi I appreciate your input, i'm still getting the hang of programming and you are right I should have tested that the variables actually were what I meant them to be. I am not used to the error messages that python throws and so I assumed the error was due to something entirely different. Ill try to improve in the future!

Germano · Accepted Answer · 2014-04-02 10:44:48Z

2

There is an error in your code. You are passing 1 as the first argument to velocity_field which in turn passes it to LHS as the second argument (y). Lastly, you call __getitem__ on y by doing y[0], and that raises the exception.

Moreover, there is a syntax error as you assign the result to return.

answered Apr 2, 2014 at 10:44

Germano

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