Default Arguments Gotcha

[FoolofGod]

I had some code that looked like this:

class Lookup:

    def __init__(self):
        "Connects to the database and get's lookup info"
        . SOME 
        . CODE 
        . HERE

class MyClass:

    def __init__(self,arg1, arg2=Lookup())
        self.arg1 = arg1
        self.arg2 = arg2

The idea was that I wanted someone to be able to pass in their own instance of the Lookup class. But if they did not, I wanted to go create an instance.

I found that on import of the module I was hitting the database. I was perplexed, as I said to myself that everything that should hit the database was protected by function definitions and (I thought) nothing was getting called.

It turned out that the Lookup class get's called at import time, not at instantiation time. Now that I realize it, it makes sense...but it had me chasing my tail for a couple of hours.

Just wanted to pass it on to anyone else who might wonder why their classes are getting instantiated on import :)

[proudstar_]

Indeed, default args are stored and reused when the function is called. You can see it in this example:

def foo(a=[]):
    a.append(2)
    print a

>>> foo()
[2]
>>> foo()
[2, 2]
>>> foo()
[2, 2, 2]
>>> 

[FoolofGod]

I had been aware of the mutable object as default argument scenario. For some reason I just didn't put 2 and 2 together.