Basic python usage, calling a function

Unlike many other programming languages, such as Java, Python does not require the main method to be inside a class. Even more, Python does not need to have a main method defined: it runs the whole file as an application. In your original post, Python performs this actions:

• Define the method singleNumber as the code it holds.
• Define the method main as the code it holds.
• Save this two methods inside the class Solution.
• No more lines to run, therefore ends application.

To make the application correct, you must write it as follows:

class Solution(object):
    def singleNumber(self, nums):
        for i in range(0,len(nums),2):
            if (i != len(nums) - 1) and (nums[i] != nums[i+1]):
                print (nums[i])
            elif i == len(nums) - 1:
                print (nums[i])

if __name__=='__main__':
    nums=[1,1,2,2,3]
    s=Solution()
    s.singleNumber(nums)
    print('done')

You may wonder why the line if __name__=='__main__':; every file contains the variable __name__ implicitly defined and its value depends if you're running the file directly or it is imported. On the first case, the assignment __name__='__main__' is done, on the second case the __name__ variable is assigned the name of the file itself; this gives you an idea whether this file is the main or not.

You may also discard the Solution class and promote the singleNumber method to a module method.

Move your main function outside of the class, and then specifically execute it:

class Solution(object):
    def singleNumber(self, nums):

        for i in range(0,len(nums),2):
            if (i != len(nums) - 1) and (nums[i] != nums[i+1]):
                print (nums[i])
            elif i == len(nums) - 1:
                print (nums[i])

def main():
    nums=[1,1,2,2,3]
    s=Solution()
    s.singleNumber(nums)
    print('done')

main()

Output:

3
done

Another possibility is to make it a separate function at all: drop the def main line, un-indent the four lines of that code, and run it from the top level.