Comments on: Default arguments in Python: two easy blunders

By: Python pitfall: Passing Mutable Objects as Default Args « Parerga und Paralipomena

Mon, 18 May 2009 15:18:37 +0000

[...] in default arguments are calculated when the function is defined, not when it’s called (read this as well). The solution is to pass None as the default value, and then add a line ‘listaexit = [...]

By: SEO

SEO — Wed, 17 Sep 2008 21:59:04 +0000

I had similar problems to the second one using delayed javascript functions. Thanks for the heads up.

By: Omer

Omer — Sun, 14 Sep 2008 02:07:27 +0000

Omg. Thanks so much! I’ve been struggling with this same problem for like an hour!

By: Denis

Denis — Tue, 01 Jul 2008 18:38:41 +0000

I just hit Blunder#2. I’m pretty sure it isn’t the first time, but I felt compelled to get to the root of it once and for all. In the end, I feel disappointed with this violation of the “you get what you expect” principle that generally pervades the realm of Pythonese.

If I had wanted a class instance variable, I would have declared one, I’m not shy about it, honest. Is this truly not considered a bug in the interpreter?

By: garth

garth — Thu, 12 Jun 2008 05:51:46 +0000

I put the open() with the rest of the code that does the work; I find it easier to read. Great use of an “and” in that last line, too.

By: swr

swr — Sun, 01 Jun 2008 20:50:30 +0000

Jay said,
ON MAY 9TH, 2008 AT 12:57 PM

file = None

try:
file = open(’/tmp/somefile’)
finally:
file and file.close()

why putting ‘open’ into the try-finally block?
If it fails there is nothing to close anyway.
I was repeating such construction many times until I realized it’s a bit illogical.

By: garth

garth — Thu, 15 May 2008 01:24:32 +0000

I just batch-approved all these excellent comments. Thanks in particular for all the reminders to use the ‘or’ operator. Cheers!

By: Jasko

Jasko — Mon, 12 May 2008 17:26:50 +0000

How about something like this to handle both cases?

def report(when=time.time):
… if hasattr(when, “__call__”):
… print when()
… else:
… print when

>>> report()
1210613185.87
>>> report(”Now”)
Now

Though regarding your second example — yowza! I can see a sleepless night worrying about that one!

By: me

me — Sun, 11 May 2008 10:58:18 +0000

the report example would be simpler with

def report(when=None):
when = when or time.time()
print when

By: David

David — Sat, 10 May 2008 13:06:33 +0000

For the report function, I would use this implementation :
>>> def report(when=time.time):
… print when()

Note that no parenthesis are used in the default parameter.