How to round a number to n decimal places?

Question

I want, what I enter in the entry field should be automatic rounded to n decimal points.

import Tkinter as Tk

root = Tk.Tk()

class InterfaceApp():
    def __in         


        

Answer 1

You do not get the expected result because when you run a.set(round(self.entry, 2)) inside initialize() , the value of self.entry.get() is always 0 (the default value after creation)

You rather need to attach a callback to a button widget on which, after pressing, the behavior you are looking for will be executed:

import Tkinter as Tk

root = Tk.Tk()

class InterfaceApp():

    def __init__(self,parent):
        self.parent = parent
        root.title("P")
        self.initialize()

    def initialize(self):
        frPic = Tk.Frame(bg='', colormap='new')
        frPic.grid(row=0, column=0)
        self.a = Tk.DoubleVar()
        self.entry = Tk.Entry(frPic, textvariable=self.a)
        self.entry.insert(Tk.INSERT,0)
        self.entry.grid(row=0, column=0)
        # Add a button widget with a callback
        self.button = Tk.Button(frPic, text='Press', command=self.round_n_decimal)
        self.button.grid(row=1, column=0)
    # Callback    
    def round_n_decimal(self):      
       self.a.set(round(float(self.entry.get()), 2))

if __name__ == '__main__':
    app = InterfaceApp(root)
    root.mainloop()

Answer 2

I suppose what you want is not to round the float value itself, you want to show a float value with a precision of n decimal points. Try this:

>>> n = 2
>>> '{:.{}f}'.format( 3.1415926535, n )
'3.14'
>>> n = 3
>>> '{:.{}f}'.format( 3.1415926535, n )
'3.142'

Note: in your code you try to round self.entry .i. e. you try to round an instance of type Tk.Entry. You should use self.entry.get() which supplies you with a string.

If you are not familiar with this kind of string formatting I use look here.