eval() function in object-oriented programming

Question

Here's the thing: My program is a GUI-based calculator written in python/GTK. In the first version I didn't used classes, so here is a piece of the old code:

def show(result):
     textbox3.set_text( str(result) )
     (...) # Update counters, etc.

def on_button_pressed(*args):
    input_user = inputbox.get_text() 
    (...) # parsing of the input
    show( eval( input_user ) )

For instance, if I type on inputbox "12+3" and press the button, textbox3 show the result "15".

I've modified my project to use OOP. Here's the modified code:

class App:
    (...)

    def show(self,result):
         self.textbox3.set_text( str(result) )
         (...) # Update counters, etc.

    def on_button_pressed(self,*args):
        input_user = self.inputbox.get_text() 
        (...) # parsing of the input
        print input_user
        self.show( eval( input_user ) )

With this code, the textbox3 show the result "<app.c12app.App instance at 0x272e128>". Which mistake am I making here?

P.S.: The real code is too large, the parsing section is about 50 lines large. I added a line print input_user to proof that the parsing don't overwrite the variable input_user. The console prints the expression parsed (a string) correctly. But when I use the eval function in this string it returns an object, instead the numeric value of the expression.

What is input_user? Paste the rest of your on_button_pressed method. I have a feeling you may be overwriting input_user somewhere. — Blender
– Blender, Commented May 21, 2013 at 0:17
you are not calling the method self.show() here. You are still calling a function show(); no idea what might invoke App().show(), but it is not in the code you show here. — Martijn Pieters
– Martijn Pieters, Commented May 21, 2013 at 0:23
The output you show look like you were instead passing in self explicitly somewhere: self.show(self). — Martijn Pieters
– Martijn Pieters, Commented May 21, 2013 at 0:24
In other words: the code you added in the question wouldn't work, and if it did, would not show the result you shared with us. Please share your real code. — Martijn Pieters
– Martijn Pieters, Commented May 21, 2013 at 0:24
The real code is too large, the parsing section is about 50 lines large. I modified the self.show (it was wrong, indeed) and added a line print input_user to proof that the parsing don't overwrite the variable input_user. The console prints the expression parsed (a string) correctly. But when I use the eval function in this string it returns an object, instead the numeric value of the expression. — David Sousa
– David Sousa, Commented May 21, 2013 at 1:00

mrKelley · Accepted Answer · 2013-05-21 01:03:42Z

1

It could be a couple things. First off, don't do class App(): in your definition, do class App:, and make sure you instantiate like foo = App() and not foo = App.

answered May 21, 2013 at 1:03

mrKelley

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2 Comments

mrKelley Over a year ago

Ok. This string <app.c12app.App instance at 0x272e128> represents an instance of one of your App objects (in memory), and the code/behavior can't really lie. Somehow you are not evaluating something as simple as 12+3. Since you're parsing for 50 lines anyway, you may as well find a way to get rid of eval(). It makes your program easily breakable, or worse...

David Sousa Over a year ago

I've found my error. I should define the show function as def show(self, dummy, result): Independently if I pass just one argument to the function (like in self.show(result), it seems python passes two "invisible arguments": the first is "self", the instance of the class and the second is the method who called the function (the function on_button_pressed or the button objetct, i'm not sure). That's a bit confusing...

David Sousa · Accepted Answer · 2013-05-21 01:39:46Z

0

I've found my error. I should define the show function as

def show(self, dummy, result):

Independently if I pass just one argument to the function (like in self.show(result), it seems python passes two "invisible arguments": the first is "self", the instance of the class and the second is the method who called the function (the function on_button_pressed or the button objetct, i'm not sure). That's a bit confusing...

the first and the second arguments are useless for me, but apparently I'm forced to declare them.

answered May 21, 2013 at 1:39

David Sousa

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