當(dāng)引擎在“... command = ...”行分配值時(shí)，引擎會(huì)評(píng)估函數(shù)的結(jié)果。
“命令”期望返回一個(gè)函數(shù)，這就是為什么使用lambda可以完成這項(xiàng)工作，因?yàn)樗鼊?chuàng)建了一個(gè)在評(píng)估期間返回“命令”的異常函數(shù)。您也可以編寫自己的功能，它也可以完成這項(xiàng)工作。
這是一個(gè)lambda和沒(méi)有l(wèi)ambda的例子：
#!/usr/bin/python
# coding=utf-8
from Tkinter import *
# Creation de la fenêtre principale (main window)
Mafenetre = Tk()
res1 = StringVar()
res2 = StringVar()
def isValidInput(obj):
if hasattr(obj, 'get') and callable(getattr(obj, 'get')):
return TRUE
return FALSE
# stupid action 2 (return 12 on purpose to show potential mistake)
def action1(*arguments):
print "action1 running"
for arg in arguments:
if isValidInput(arg):
print "input value: ", arg.get()
res1.set(arg.get())
else:
print "other value:", arg
print "\n"
return 12
# stupid action 2
def action2(*arguments):
print "action2 running"
a = arguments[0]
b = arguments[1]
if isValidInput(a) and isValidInput(b):
c = a.get() + b.get()
res2.set(c)
print c
print "\n"
# a stupid workflow manager ordered by name
def start_tasks(*arguments, **keywords):
keys = sorted(keywords.keys())
for kw in keys:
print kw, "plugged "
keywords[kw](*arguments)
# valid callback wrapper with lambda
def action1_callback(my_input):
return lambda args=[my_input]: action1(*args)
# valid callback wrapper without lambda
def action1_callback_nolambda(*args, **kw):
def anon():
action1(*args)
return anon
# first input string
input1 = StringVar()
input1.set("delete me...")
f1 = Entry(Mafenetre, textvariable=input1, bg='bisque', fg='maroon')
f1.focus_set()
f1.pack(fill="both", expand="yes", padx="5", pady=5)
# failed callback because the action1 function is evaluated, it will return 12.
# in this case the button won't work at all, because the assignement expect a function
# in order to have the button command to execute something
ba1 = Button(Mafenetre)
ba1['text'] = "show input 1 (ko)"
ba1['command'] = action1(input1)
ba1.pack(fill="both", expand="yes", padx="5", pady=5)
# working button using a wrapper
ba3 = Button(Mafenetre)
ba3['text'] = "show input 1 (ok)"
# without a lambda it is also working if the assignment is a function
#ba1['command'] = action1_callback_nolambda(input1)
ba3['command'] = action1_callback(input1)
ba3.pack(fill="both", expand="yes", padx="5", pady=5)
# display result label
Label1 = Label(Mafenetre, text="Action 1 result:")
Label1.pack(fill="both", expand="yes", padx="5", pady=5)
# display result value
resl1 = Label(Mafenetre, textvariable=res1)
resl1.pack(fill="both", expand="yes", padx="5", pady=5)
# second input string
input2 = StringVar()
f2 = Entry(Mafenetre, textvariable=input2, bg='bisque', fg='maroon')
f2.focus_set()
f2.pack(fill="both", expand="yes", padx="5", pady=5)
# third test without wrapper, but making sure that several arguments are well handled by a lambda function
ba2 = Button(Mafenetre)
ba2['text'] = "execute action 2"
ba2['command'] = lambda args=[input1, input2], action=action2: start_tasks(*args, do=action)
ba2.pack(fill="both", expand="yes", padx="5", pady=5)
# display result label
Label2 = Label(Mafenetre, text="Action 2 result:")
Label2.pack(fill="both", expand="yes", padx="5", pady=5)
# display result value
resl2 = Label(Mafenetre, textvariable=res2)
resl2.pack(fill="both", expand="yes", padx="5", pady=5)
Mafenetre.mainloop()
