Initialize a string variable in Python: "" or None?

Question

Suppose I have a class with a string instance attribute. Should I initialize this attribute with "" value or None? Is either okay?

def __init__(self, mystr="")    self.mystr = mystr 

or

def __init__(self, mystr=None)    self.mystr = mystr 

Edit: What I thought is that if I use "" as an initial value, I "declare" a variable to be of string type. And then I won't be able to assign any other type to it later. Am I right?

Edit: I think it's important to note here, that my suggestion was WRONG. And there is no problem to assign another type to a variable. I liked a comment of S.Lott: "Since nothing in Python is "declared", you're not thinking about this the right way."

Answer 1

If not having a value has a meaning in your program (e.g. an optional value), you should use None. That's its purpose anyway.

If the value must be provided by the caller of __init__, I would recommend not to initialize it.

If "" makes sense as a default value, use it.

In Python the type is deduced from the usage. Hence, you can change the type by just assigning a value of another type.

>>> x = None >>> print type(x) <type 'NoneType'> >>> x = "text" >>> print type(x) <type 'str'> >>> x = 42 >>> print type(x) <type 'int'> 

Answer 2

Another way to initialize an empty string is by using the built-in str() function with no arguments.

str(object='')

Return a string containing a nicely printable representation of an object.

...

If no argument is given, returns the empty string, ''.

In the original example, that would look like this:

def __init__(self, mystr=str())    self.mystr = mystr 

Personally, I believe that this better conveys your intentions.

Notice by the way that str() itself sets a default parameter value of ''.