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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."

2012/03/01
1
72
3/1/2012 12:56:54 AM

Accepted Answer

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'>
2009/09/24
79
9/24/2009 12:32:46 PM

None is used to indicate "not set", whereas any other value is used to indicate a "default" value.

Hence, if your class copes with empty strings and you like it as a default value, use "". If your class needs to check if the variable was set at all, use None.

Notice that it doesn't matter if your variable is a string initially. You can change it to any other type/value at any other moment.

2009/09/09

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 ''.

2013/09/08

It depends. If you want to distinguish between no parameter passed in at all, and an empty string passed in, you could use None.

2009/09/09

Either might be fine, but I don't think there is a definite answer.

  • If you want to indicate that the value has not been set, comparing with None is better than comparing with "", since "" might be a valid value,
  • If you just want a default value, "" is probably better, because its actually a string, and you can call string methods on it. If you went with None, these would lead to exceptions.
  • If you wish to indicate to future maintainers that a string is required here, "" can help with that.

Complete side note:

If you have a loop, say:

def myfunc (self, mystr = ""):
    for other in self.strs:
        mystr = self.otherfunc (mystr, other)

then a potential future optimizer would know that str is always a string. If you used None, then it might not be a string until the first iteration, which would require loop unrolling to get the same effects. While this isn't a hypothetical (it comes up a lot in my PHP compiler) you should certainly never write your code to take this into account. I just thought it might be interesting :)

2009/09/09

Either is fine, though None is more common as a convention - None indicates that no value was passed for the optional parameter.

There will be times when "" is the correct default value to use - in my experience, those times occur less often.

2009/09/09

Source: https://stackoverflow.com/questions/1398164
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