Praveen Singh wrote:
In function-
"Default value is *evaluated only once*.This makes different when the
default is a mutable object such as a list, dictionary or instance of most
classes."
I am not getting it properly-evaluated once?? different behaviour???--
please explain this.
Look at an example:
>>> import time
>>> def test(t=time.asctime()):
... print t, "***", time.asctime()
...
>>> time.sleep(30) # wait a little bit
>>> test()
Sat Oct 22 04:17:08 2011 *** Sat Oct 22 04:17:57 2011
>>> time.sleep(30) # wait a little bit longer
>>> test()
Sat Oct 22 04:17:08 2011 *** Sat Oct 22 04:18:46 2011
Notice that the first time printed, using the default value, is the
same. The default value for t is assigned once, and not calculated
again. Since t is a string, it is immutable and can never change.
The same thing occurs when you use a mutable object like a list or a
dict. The default value is assigned once, and once only. But notice that
you can modify the default value, say by appending to it:
>>> def test(x=[]):
... print x, id(x)
... x.append(1)
...
>>> test()
[] 3085600236L
>>> test()
[1] 3085600236L
>>> test()
[1, 1] 3085600236L
It is the same default list every time, but the *contents* of the list
are changing.
--
Steven
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