What is the purpose of "None" in event=None with tkinter ?

[alan9979]

In the following code, I try with and without the "=None" on line 30 (def changeCouleurs(self,event=None):), but there's no difference. I would like to understand in which case you use event=None.
Thanks for any help

class Application(object):
	def __init__ (self):
		"""Construteur de la fenêtre principale"""
		self.fen=Tk()
		self.fen.title('Code des couleurs')
		self.dessineResistance()
		self.dessineBouton()
		self.codeCouleur = ['black','brown','red','orange','yellow',\
							'green','blue','purple','grey','white']
	def dessineResistance(self):
		self.can=Canvas(self.fen,bg='ivory',width =500,height=300)
		self.can.grid(row = 1, column = 1, columnspan=3)
		self.can.create_line(10,150,490,150,width = 4)
		self.can.create_rectangle(100,100,400,200,fill='light grey')
		self.ligne=[]
		for i in range(3):
			self.ligne.append(self.can.create_rectangle(130+i*35,\
									100,145+i*35,200,fill='black'))

	def dessineBouton(self):
		Label(self.fen,text ='Entrez la valeur de la résitance, en ohms :').\
											grid(row=2,column=1,columnspan=3)
		#Button(self.fen,text='Montrer',command=self.changeCouleurs).grid(row=3,column=1)
		self.entree=Entry(self.fen,width=14)
		self.entree.grid(row=3,column=2)
		self.entree.bind("<Return>",self.changeCouleurs)
		Button(self.fen,text='Quitter',command=self.fen.quit).\
											grid(row=3,column=3)

	def changeCouleurs(self,event=None):
		"""Mettre event dans la fonction si elle est appelée avec un bind"""
		#test avec enent=None : marche pareil
		self.valEntree=self.entree.get()
		try:
			val=float(self.valEntree)
		except:
			erreur = 1
		else:
			erreur = 0
		if erreur == 1 or val >1e11 or val < 1:
			self.signalErreur()
		else :
			li =[0]*3 # liste des 3 codes à afficher
			if val >= 10:
				logv = int(log10(val)) # partie entière du logarithme
				ordgr = 10**logv # ordre de grandeur
				v_temp = round(val/ordgr*10)
				# extraction du premier chiffre significatif :
				li[0] = int(v_temp/10) # partie entière
				li[1] = v_temp- 10*li[0] # partie décimale
				li[2] = logv -1
			else:
				v_temp=round(val,1)
				li[0]=0
				li[1]=int(v_temp)
				li[2]=int(10*(v_temp-li[1]))
			for i in range(3):
				self.can.itemconfig(self.ligne[i],fill=self.codeCouleur[li[i]])

	def signalErreur(self):
		self.entree.config(bg='red')
		self.entree.after(1000,self.videEntree)

	def videEntree(self):
		self.entree.config(bg='white')
		self.entree.delete(0,len(self.valEntree))
		for i in range(3):
			self.can.itemconfig(self.ligne[i],fill='black')




from tkinter import *	
from math import log10	
appli = Application()
appli.fen.mainloop()
appli.fen.destroy()

[ichabod801]

The event parameter is not used in the changeCouleurs method. However, that method is bound to an event on line 26. As such, it will be passed an event parameter. They defined it with None as a default. As you noted, the default has no effect on the code. However, it would allow you to call changeCouleurs() without a parameter elsewhere in the application. They don't do that in this code, but maybe they have plans to do it later.

So the '=None' does nothing, except allow for some flexibility that isn't currently being used.

[alan9979]

Thank you Ichabod801, it helps