function adding numbers from a list in Python 3.6

Lots of things wrong here.

• sumat has not been assigned a value hence you cannot execute the line "sumat+=numero"
• you redefine the argument numeros with the line "numeros=[]" -- the indentation is crucial in python
• same with your for loop -- due to indentation, it is part of the definition of the function sumanu

What are you trying to do?

Are you trying to implement the built-in function sum in module builtins?
Here's a guess as to what you are trying to do:

def sumanu(numeros):
    sumat = 0
    for numero in numeros:
        sumat += numero

    return sumat

x = [1, 3, 5, 7, 9]
print(sumanu(x))

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To add, when using numeros=[], you reuse the name numeros for another purpose. In other words, you throw away what you passed via the argument of that name, and you assign the variable the empty list. This way, you always get zero.

Flabio: your answer does not return any error. However it only takes into account one argument. So if I type  x=[2,5,7]
it only returns "2".

This function is intended to add all numbers from a given list. I would prefer to type the numbers directly into the funtion instead of assigning them to a variable, but not sure if it is possible, For example once I call the function,  sumanu(2,5,7) it could return the answer: "14".

You have to keep return sumat exactly below the for (the same indentation. You probably have it indented the same way like sumat += numero. This way it returns immediately after the first addition, and both the loop and the function are exited prematurely.

In other programming languages, indentation is just a guide. Python requires proper indentation.

Again thanks!

I just had to indent one more space "sumat+=numero" than you suggested. If "sumat+=numero" goes exactly  below "ro" from the previous line, works perfect!

had to indent one more space "sumat+=numero" than you suggested

Are you mixing spaces and tabs? You should not! I recommend using 4 spaces as a standard.