Calling a function from a class in python

Question

I'm trying to call a function from a class in python. This is my code:

class JCMT:
    def be(self):
       if self > 330000:
         return 0.64
       else:
         return 0.69
    def bs(self):
       if self > 330000:
         return 14
       else:
         return 20 

f = 220258

print JCMT.bs(f)

I'm getting an error of TypeError: unbound method bs() must be called with JCMT instance as first argument (got int instance instead)

Could someone tell me where I'm going wrong?

score 5 · Answer 1 · edited May 23 '17 at 11:43

5

There were a few problems, I addressed them with comments:

class JCMT:

    # Add parameter
    def bs(self, n):
       if n > 330000:
         return 14
       else:
         return 20

f = 220258

# Make a new instance
j = JCMT()

# Call now, and pass variable
print j.bs(f)

The self part in the method signature is unique. You can read more about it in this answer.

edited May 23 '17 at 11:43

Community

1
1

answered Apr 28 '15 at 20:52

Tyler

17,669
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Malik Brahimi · Answer 2 · 2015-04-28T20:56:06.340

5

Add a static method decorator right above the function to denote a static function called on the class:

@staticmethod

Problem is your self parameter corresponds to an instance object which is implicitly passed during function invocation and thus an error is thrown when a class is supplied instead. You must remove the self argument and use an argument to denote the numerical argument.

edited Apr 28 '15 at 20:56

answered Apr 28 '15 at 20:52

Malik Brahimi

16,341
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70

this is also a great answer :) – Joran Beasley Apr 28 '15 at 20:53
2

This is the right way. On top, I would rename the argument from **self** to something else so as to avoid confusion with regular instance methods. – honza_p Apr 28 '15 at 20:54
2

It's quite possible that this was supposed to be a regular instance method, and the OP simply forgot (or is unaware of how) to instantiate an object. – TigerhawkT3 Apr 28 '15 at 20:55

score 0 · Answer 3 · answered Apr 28 '15 at 21:21

Althout the @staticmethod decorator already provided in an answer solves this particular problem, if in the future you wish to use the class method and retain self, you can also use the @classmethod decorator to call the method directly without instantiating the class first.

class JCMT:
    @classmethod
    def be(self, n):
        if n > 330000:
            return 0.64
        else:
            return 0.69
    @classmethod
    def bs(self, n):
        if n > 330000:
            return 14
        else:
            return 20 

f = 220258

print JCMT.bs(f)
20

Calling a function from a class in python

3 Answers3