Python "self" is not defined in function arguments list

Question

Is there any way to do the following?

def getTables(self, db = self._dbName):
    //do stuff with db    
    return func(db)

I get a "self" is not defined. I could do this...

def getTables(self, db = None):
    if db is None:
        db = self._db
    //do stuff with db
    return func(db)

... but that is just annoying.

You want to set a default value for an argument based on other arguments? — Lix
– Lix, Commented Aug 5, 2014 at 15:57
the correct way to do it is: def getTables(self, db = None): — levi
– levi, Commented Aug 5, 2014 at 15:57
Short answer: no. None is the correct default here. self doesn't exist until inside the method. — jonrsharpe
– jonrsharpe, Commented Aug 5, 2014 at 15:58

Martijn Pieters · Accepted Answer · 2014-08-05 16:37:31Z

8

Function signatures are evaluated when the function is defined, not when the function is called.

When the function is being defined there are no instances yet, there isn't even a class yet.

To be precise: the expressions for a class body are executed before the class object is created. The expressions for function defaults are executed before the function object is created. At that point, you cannot create an instance to bind self to.

As such, using a sentinel (like None) is really your only option. If db is never falsey you can simply use:

def getTables(self, db=None):
    db = db or self._db

edited Aug 5, 2014 at 16:37

answered Aug 5, 2014 at 15:59

Martijn Pieters

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1 Comment

Charles Duffy Over a year ago

+1 -- a much better answer than mine, since understanding this is important to understanding (for instance) why def foo(bar={}) is Evil And Wrong (sharing a single, mutable {} object, rather than creating a new dict on every call).

Lix · Accepted Answer · 2014-08-05 16:00:02Z

1

Perhaps this onle-liner version will be slightly less "annoying":

def getTables( self, db = None):
    db = self._db if db is None else db
    ...

You simply can not set a default value for an argument based upon the value of a another argument because the argument you want to base your default value on hasn't been defined yet.

answered Aug 5, 2014 at 16:00

Lix

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2 Comments

dylnmc Over a year ago

I just don't want the if statement in general. I want it in the function parameters list

Lix Over a year ago

@Dylan - ah yes... but what about what the python interpreter wants? ;)

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