Let's say I have a class that has a member called data which is a list.
I want to be able to initialize the class with, for example, a filename (which contains data to initialize the list) or with an actual list.
What's your technique for doing this?
Do you just check the type by looking at __class__?
Is there some trick I might be missing?
I'm used to C++ where overloading by argument type is easy.
Excellent question. I've tackled this problem as well, and while I agree that "factories" (class-method constructors) are a good method, I would like to suggest another, which I've also found very useful:
Here's a sample (this is a read method and not a constructor, but the idea is the same):
def read(self, str=None, filename=None, addr=0):
""" Read binary data and return a store object. The data
store is also saved in the interal 'data' attribute.
The data can either be taken from a string (str
argument) or a file (provide a filename, which will
be read in binary mode). If both are provided, the str
will be used. If neither is provided, an ArgumentError
is raised.
"""
if str is None:
if filename is None:
raise ArgumentError('Please supply a string or a filename')
file = open(filename, 'rb')
str = file.read()
file.close()
...
... # rest of code
The key idea is here is using Python's excellent support for named arguments to implement this. Now, if I want to read the data from a file, I say:
obj.read(filename="blob.txt")
And to read it from a string, I say:
obj.read(str="\x34\x55")
This way the user has just a single method to call. Handling it inside, as you saw, is not overly complex
You probably want the isinstance builtin function:
self.data = data if isinstance(data, list) else self.parse(data)