Is it feasible to execute multiple statements in a lambda? The app_data dictionary in my controller has a property "listbox" that stores a selected listbox value. A button on SelectPage has a lambda command that changes frames to NextPage.
Can the frame switch and app_data set statements both occur in a double-lambda? If so, what might this look like? Or do these operations have to occur separately (i.e. in a button click event)?
Update #1:
Issue isolated to lambda expression since just putting a constant (i.e. 1) in like .set(1) does not save either.
Update #2:
Using a lambda with doStuff(controller) solves the problem somewhat, but the listbox reference throws an error:
value = self.listbox.get(self.listbox.curselection()) AttributeError: 'SelectPage' object has no attribute 'listbox'
button:
button1 = ttk.Button(self, text='Next Page',
command=lambda:self.doStuff(controller))
doStuff():
def doStuff(self,controller):
controller.show_frame(NextPage)
controller.app_data["listbox"].set(1)
value = self.listbox.get(self.listbox.curselection())
print(value)
print("do Stuff successful")
Full Code:
from tkinter import *
from tkinter import ttk
class MyApp(Tk):
# Controller class
def __init__(self):
Tk.__init__(self)
# App data in controller
self.app_data = {"listbox": StringVar(),
"entry": StringVar(),
}
container = ttk.Frame(self)
container.pack(side="top", fill="both", expand = True)
self.frames = {}
for F in (SelectPage, NextPage):
frame = F(container, self)
self.frames[F] = frame
frame.grid(row=0, column=0, sticky = NSEW)
self.show_frame(SelectPage)
def show_frame(self, cont):
frame = self.frames[cont]
frame.tkraise()
def get_page(self,classname):
for page in self.frames.values():
if str(page.__class__.__name__) == classname:
return page
return None
class SelectPage(ttk.Frame):
def __init__(self, parent, controller):
self.controller = controller
ttk.Frame.__init__(self, parent)
ttk.Label(self, text='Select Page').grid(padx=(20,20), pady=(20,20))
listbox = Listbox(self,exportselection=0)
listbox.grid()
for item in [0,1,2,3,4,5]:
listbox.insert(END, item)
print (item)
entry1 = ttk.Entry(self, textvariable=self.controller.app_data["entry"], width=8)
entry1.grid()
button1 = ttk.Button(self, text='Next Page',
command=lambda:self.doStuff(controller)) # something like this lambda concept
button1.grid()
def doStuff(self,controller):
controller.show_frame(NextPage)
controller.app_data["listbox"].set(1)
value = self.listbox.get(self.listbox.curselection())
print(value)
print("do Stuff successful")
class NextPage(ttk.Frame):
def __init__(self, parent, controller):
self.controller = controller
ttk.Frame.__init__(self, parent)
ttk.Label(self, text='Next Page').grid(padx=(20,20), pady=(20,20))
button1 = ttk.Button(self, text='Select Page',
command=lambda: controller.show_frame(SelectPage))
button1.grid()
button2 = ttk.Button(self, text='press to print', command=self.print_it)
button2.grid()
def print_it(self):
value = self.controller.app_data["listbox"].get()
print ('The value stored in StartPage some_entry = ' + str(value))
value = self.controller.app_data["entry"].get()
print ('The value stored in StartPage some_entry = ' + str(value))
app = MyApp()
app.title('Multi-Page Test App')
app.mainloop()
2 Answers
Your original lambda didn't work because show_frame(...) returns None, which short-circuits the and so the remaining expression doesn't execute. Just change it to an or. Since the event caller doesn't care what you return, you could also create a two item tuple for the two subexpressions. There are other problems, such as self.listbox doesn't exist, which I fixed, but others remain.
As a pedantic aside, lambda only accepts an expression, which is a subset of a statement.
self.listbox = listbox
button1 = ttk.Button(self, text='Next Page',
command=lambda: controller.show_frame(NextPage) or self.controller
.app_data["listbox"]
.set(self.listbox.get(self.listbox.curselection()))) # something like this lam
Is it feasible to execute multiple statements in a lambda?
No. In general, you should avoid using lambda at all, and when you must use it you need to keep it as brief as possible. If you need to call more than a single function, create a new function that calls the other functions.
Also, if you're passing in an argument that is just an attribute of the object, you don't need lambda at all. I think the following code is much easier to read, much easier to understand, much easier to write, and thus much easier to maintain:
button1 = ttk.Button(self, text='Next Page',command=self.doStuff)
...
def doStuff(self):
self.controller.show_frame(NextPage)
self.controller.app_data["listbox"].set(1)
