[Tutor] Tkinter wierdness

[Magnus Lycka]

At 12:42 2002-12-10 -0600, Isaac Hall wrote:
>P.S. the inevitable follow-up question:  why must this be done?  if I wish
>to call a function with some arguments, how is this done?

Because "command=function" means "function is the command you should
run". "command=function()" means "The result value from function is
the command you should run".

Look here:
 >>> def hw(aString="world"):
...     return "Hello %s" % aString
...
 >>> print hw()
Hello world
 >>> print hw
<function hw at 0x015E9900>
 >>> command = hw
 >>> command
<function hw at 0x015E9900>
 >>> command()
'Hello world'
 >>> command('Moon')
'Hello Moon'
 >>> command = hw()
 >>> command
'Hello world'
 >>> command()
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
TypeError: 'str' object is not callable

See the difference? When you bind a command in Tkinter, you tell
the computer what to run at a later time. You must hand over something
that can run, right?

But look here!
 >>> class hw:
...     def __init__(self, greeting='hello'):
...             self.greeting = greeting
...     def __call__(self, who='world'):
...             return "%s %s" % (self.greeting, who)
...
 >>> command = hw('hi') # Run __init__
 >>> command('moon')    # Run __call__
'hi moon'


-- 
Magnus Lycka, Thinkware AB
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