What does "bound method" error mean when I call a function?

Question

I am creating a word parsing class and I keep getting a

bound method Word_Parser.sort_word_list of <__main__.Word_Parser instance at 0x1037dd3b0> 

error when I run this:

class Word_Parser:     """docstring for Word_Parser"""     def __init__(self, sentences):         self.sentences = sentences      def parser(self):         self.word_list = self.sentences.split()      def sort_word_list(self):         self.sorted_word_list = self.word_list.sort()      def num_words(self):         self.num_words = len(self.word_list)  test = Word_Parser("mary had a little lamb") test.parser() test.sort_word_list() test.num_words() print test.word_list print test.sort_word_list print test.num_words 

Answer 1

There's no error here. You're printing a function, and that's what functions look like.

To actually call the function, you have to put parens after that. You're already doing that above. If you want to print the result of calling the function, just have the function return the value, and put the print there. For example:

print test.sort_word_list() 

On the other hand, if you want the function to mutate the object's state, and then print the state some other way, that's fine too.

Now, your code seems to work in some places, but not others; let's look at why:

• parser sets a variable called word_list, and you later print test.word_list, so that works.
• sort_word_list sets a variable called sorted_word_list, and you later print test.sort_word_list—that is, the function, not the variable. So, you see the bound method. (Also, as Jon Clements points out, even if you fix this, you're going to print None, because that's what sort returns.)
• num_words sets a variable called num_words, and you again print the function—but in this case, the variable has the same name as the function, meaning that you're actually replacing the function with its output, so it works. This is probably not what you want to do, however.

(There are cases where, at first glance, that seems like it might be a good idea—you only want to compute something once, and then access it over and over again without constantly recomputing that. But this isn't the way to do it. Either use a @property, or use a memoization decorator.)

Answer 2

This problem happens as a result of calling a method without brackets. Take a look at the example below:

class SomeClass(object):     def __init__(self):         print 'I am starting'      def some_meth(self):         print 'I am a method()'  x = SomeClass() ''' Not adding the bracket after the method call would result in method bound error ''' print x.some_meth ''' However this is how it should be called and it does solve it ''' x.some_meth()