16 Replies Latest reply on Dec 15, 2012 6:06 AM by wallarug

    Creating Python 2.7 Class

    wallarug

      Hi, I am attempting to create a set of instructions that are grouped in a file under a class.  This way, the commands that I need can be called very quickly.

       

      I am having a problem, I keep getting the following error:

      [ code ]

       

      Traceback (most recent call last):

        File "<pyshell#7>", line 1, in <module>

          CMDmodule.send_when_different()

      TypeError: unbound method send_when_different() must be called with CMDmodule instance as first argument (got nothing instead)

       

      [ /code]

       

      Does anyone know how I can get the groups of commands to work by me importing the module / class?

        • Re: Creating Python 2.7 Class
          mgt6910

          Fergus, this link may point you in the right direction...

           

          http://stackoverflow.com/questions/4473184/unbound-method-f-must-be-called-with-fibo-instance-as-first-argument-got-cla

           

           

          p.s. why did  you use 512 tests to invert the motors instead of :

           

           

          def invert_mota(self):

               motor_a = 255 - motor_a

           

          def invert_motb(self):

               motor_b = 255 - motor_b

            • Re: Creating Python 2.7 Class

              >  p.s. why did  you use 512 tests to invert the motors instead of :  ...

               

              good point!

               

              One of the design goals for programming languages is that there should

              essentially never be a need for repetitive looking code.  So when you

              see something like that, you know there's probably a better way to do it.

               

              For example, using copy/paste to duplicate code is usually a bad idea.

              Instead, you should usually create a subprogram and call it multiple times.

              If you have multiple sequential calls to the same subprogram, you should

              instead use a loop.  If you have similar but not identical subprograms,

              you can combine them by adding parameters.  If you have similar

              algorithms applied to different data types, you can combine them

              using generics.  If you have similar classes, you can combine their

              common parts using inheritance from a common base class.

               

              In the case at hand, when you have a repetitive looking elsif chain,

              you can often simplify it using a switch statement.  If you still have a

              repetitive looking switch statement, you can often simplify it using

              a table lookup, either to look up a value or to look up a subprogram

              to call indirectly.  If you still have a repetitive looking table, you can

              often simplify it using a mathematical calculation.

              • Re: Creating Python 2.7 Class
                wallarug

                Malcolm Turner wrote:

                 

                Fergus, this link may point you in the right direction...

                 

                http://stackoverflow.com/questions/4473184/unbound-method-f-must-be-called-with-fibo-instance-as-first-argument-got-cla

                 

                 

                p.s. why did  you use 512 tests to invert the motors instead of :

                 

                 

                def invert_mota(self):

                     motor_a = 255 - motor_a

                 

                def invert_motb(self):

                     motor_b = 255 - motor_b

                I will have a look at this later.

                 

                And yes, I see the error in my ways now.  I wasn't sure if that would work as the first time I used it, it spat out errors.  I have fixed it now but I would still like to use a class for some other little programs that I am using.

                  • Re: Creating Python 2.7 Class
                    wallarug

                    Here is a quick update.

                     

                    i. I can import the module.

                    ii. I can bound the class

                    iii. I can call instances within the class without any errors.

                     

                    But, the instances do not seem to do anything.  See updated script below.

                     

                    I do the following...

                    >> import CMDmodule

                    >> CMD = CMDmodule.CMDmodule()

                     

                    Then I can call the instance that I want:

                    >> CMD.zero_variables()

                     

                    I would have thought that at this stage I would now have all the variables under the "def <command>:" imported but that seems not to be the case.

                    >> CMD.zero_variables()

                    >> print a

                    Traceback (most recent call last):

                      File "<pyshell#13>", line 1, in <module>

                        print b

                    NameError: name 'b' is not defined

                     

                    What am I doing wrong?

                • Re: Creating Python 2.7 Class
                  wallarug

                  New problem with each command:

                   

                  UnboundLocalError: local variable 'y' referenced before assignment

                   

                  Any ideas on how to fix it?

                    • Re: Creating Python 2.7 Class
                      wallarug

                      It seems that the problem is under the def statements.

                       

                      when I call on "invert_y_axis()" It gives me errors. same when I call on the zero_variables().

                       

                      But when I bring out the values in zero_variables() by removing them from under that statement, and moving them to when the module is imported, I can >>> print a.

                       

                      Any ideas or quick fixes?

                        • Re: Creating Python 2.7 Class
                          wallarug

                          Still no luck.  Could someone fix this code so that it works?

                           

                          There is a problem with declearing variables.

                           

                          You should see once you run it.

                           

                          Attached is the module and the test script I am using.

                           

                          Please tell me how I can fix the problem.  Nothing seems to work.

                            • Re: Creating Python 2.7 Class
                              Drew Fustini

                              Hi - I'm still learning Python myself but I was able to get it to run by making this change to adding "a = 0" at the beginning of CMDmodule.py:

                               

                              afustini@lappy486:~/Downloads$ head CMDmodule.py
                              # C:\python27\Lib
                              
                              
                              a = 0
                              '''
                              Invert Code for Arduino PWM when Digital
                              Channel is 'LOW' along with a few other
                              bits and pieces.
                              '''
                              
                              
                              ##set_zero = 0
                              

                               

                              And running it seems to be ok (no error at least):

                               

                              afustini@lappy486:~/Downloads$ python ./Test\ Program\ for\ CMDmodule.py
                              Welcome to CMDmodule.  Use Wisely. 
                              Valables set to Zero!
                              Change this value Test: 23
                              23
                              Change this value Test: 45 
                              45
                              Change this value Test: 
                              

                               

                              Based on what I was reading here:

                               

                              http://stackoverflow.com/questions/423379/using-global-variables-in-a-function-other-than-the-one-that-created-them

                               

                              However, I'm wondering is there are reason that you are using globals heavily?  I'm still developing my sense of style for python but this would seem to be very un-"pythonic" as they say. 

                                • Re: Creating Python 2.7 Class
                                  Drew Fustini

                                  I'm not sure if this will help or not, but I thought it might give an example of something I've done.  Recently I wrote a  LCD twitter display with Python.  Initially, I wrote it as a procedural program since I don't know Python that well:


                                   

                                  https://github.com/pdp7/tweetypi/blob/master/display-hashtag.py

                                   

                                  I'm still working on improving the code, but what I tried to do is pass data that won't change for a given instance of the HashTagDisplay class in the constructor (__init__) like the number of rows and columns for the LCD.

                                   

                                   

                                  The constructor then sets attributes self.rows and self.cols that the methods in the HashTagDisplay class refer to.

                                   

                                  However, I decided to pass that hashtag to search as a parameter to the HashTagDisplay.search() method which returns the results.  The HashTagDisplay.display() method then takes those results as a parameter:

                                   

                                   

                                  My main program can thus just create a new HashTagDisplay object with the appropriate LCD parameters and then call the search and display methods in a loop:

                                   

                                   

                                  I think that you should be able avoid globals by either setting attributes in an object or passing parameters to a method and utilizing the return value.

                                  • Re: Creating Python 2.7 Class
                                    wallarug

                                    Drew Fustini wrote:

                                     

                                    Hi - I'm still learning Python myself but I was able to get it to run by making this change to adding "a = 0" at the beginning of CMDmodule.py:

                                     

                                    afustini@lappy486:~/Downloads$ head CMDmodule.py
                                    # C:\python27\Lib
                                    
                                    
                                    a = 0
                                    '''
                                    Invert Code for Arduino PWM when Digital
                                    Channel is 'LOW' along with a few other
                                    bits and pieces.
                                    '''
                                    
                                    
                                    ##set_zero = 0
                                    

                                     

                                    And running it seems to be ok (no error at least):

                                     

                                    afustini@lappy486:~/Downloads$ python ./Test\ Program\ for\ CMDmodule.py
                                    Welcome to CMDmodule.  Use Wisely. 
                                    Valables set to Zero!
                                    Change this value Test: 23
                                    23
                                    Change this value Test: 45 
                                    45
                                    Change this value Test: 
                                    

                                     

                                    Based on what I was reading here:

                                     

                                    http://stackoverflow.com/questions/423379/using-global-variables-in-a-function-other-than-the-one-that-created-them

                                     

                                    However, I'm wondering is there are reason that you are using globals heavily?  I'm still developing my sense of style for python but this would seem to be very un-"pythonic" as they say. 

                                    You have seen first hand the problem I am having!

                                    Change this value Test: 23

                                    23

                                    It should have applied the formula in CMD module:

                                    def pwm_formula_test(self):

                                            global a

                                            PWM = 255

                                            aa = PWM*a

                                            aa = round(aa, 1)

                                            motor_a = aa

                                    Well... I think yours might be a bit different but you get the idea.

                                     

                                    It should have mulitpled '23' by 255 and then given the output.

                                     

                                    Any ideas why it is not working?

                                      • Re: Creating Python 2.7 Class
                                        Drew Fustini

                                        I think it must be due to some aspect of using globals.  I would think globals wouldn't be the best way to handle this anyways.  What do you think of changing pwm_formula_test() to accept a parameter and then having it return a value?

                                        afustini@lappy486:~/Downloads$ python ./Test\ Program\ for\ CMDmodule.py

                                        Welcome to CMDmodule.  Use Wisely.

                                        Valables set to Zero!

                                        Change this value Test: 25

                                        6375.0

                                        so that method in your class would be:

                                            def pwm_formula_test(self, a):

                                                PWM = 255

                                                aa = PWM*a

                                                aa = round(aa, 1)

                                                motor_a = aa

                                                return aa

                                        and you test script would have:

                                        while True:

                                            a = input("Change this value Test: ")

                                            b = CMD.pwm_formula_test(a)

                                            print b