Multiple constructors: the Pythonic way? [duplicate]

Answer 1

I'm not sure if I understood right but wouldn't this work?

def __init__(self, file=None, timestamp=0, data=[], metadata={}):
    if file:
        ...
    else:
        self.timestamp = timestamp
        self.data = data
        self.metadata = metadata

Or you could even do:

def __init__(self, file=None, timestamp=0, data=[], metadata={}):
    if file:
        # Implement get_data to return all the stuff as a tuple
        timestamp, data, metadata = f.get_data()

    self.timestamp = timestamp
    self.data = data
    self.metadata = metadata

Thank to Jon Kiparsky advice theres a better way to avoid global declarations on data and metadata so this is the new way:

def __init__(self, file=None, timestamp=None, data=None, metadata=None):
    if file:
        # Implement get_data to return all the stuff as a tuple
        with open(file) as f:
            timestamp, data, metadata = f.get_data()

    self.timestamp = timestamp or 0
    self.data = data or []
    self.metadata = metadata or {}

Answer 2

The most pythonic way is to make sure any optional arguments have default values. So include all arguments that you know you need and assign them appropriate defaults.

def __init__(self, timestamp=None, data=[], metadata={}):
    timestamp = time.now()

An important thing to remember is that any required arguments should not have defaults since you want an error to be raised if they're not included.

You can accept even more optional arguments using *args and **kwargs at the end of your arguments list.

def __init__(self, timestamp=None, data=[], metadata={}, *args, **kwards):
    if 'something' in kwargs:
        # do something

Answer 3

You can't have multiple constructors, but you can have multiple aptly-named factory methods.

class Document(object):

    def __init__(self, whatever args you need):
        """Do not invoke directly. Use from_NNN methods."""
        # Implementation is likely a mix of A and B approaches. 

    @classmethod
    def from_string(cls, string):
        # Do any necessary preparations, use the `string`
        return cls(...)

    @classmethod
    def from_json_file(cls, file_object):
        # Read and interpret the file as you want
        return cls(...)

    @classmethod
    def from_docx_file(cls, file_object):
        # Read and interpret the file as you want, differently.
        return cls(...)

    # etc.

You can't easily prevent the user from using the constructor directly, though. (If it is critical, as a safety precaution during development, you can analyze the call stack in the constructor and check that the call is made from one of the expected methods.)

Answer 4

Most Pythonic would be what the Python standard library already does. Core developer Raymond Hettinger (the collections guy) gave a talk on this, plus general guidelines for how to write classes.

Use separate, class-level functions to initialize instances, like how dict.fromkeys() isn't the class initializer but still returns an instance of dict. This allows you to be flexible toward the arguments you need without changing method signatures as requirements change.

Answer 5

What are the system goals for this code? From my standpoint, your critical phrase is but the user has to know which method he requires. What experience do you want your users to have with your code? That should drive the interface design.

Now, move to maintainability: which solution is easiest to read and maintain? Again, I feel that solution C is inferior. For most of the teams with whom I've worked, solution B is preferable to A: it's a little easier to read and understand, although both readily break into small code blocks for treatment.

Answer 6

You can't have multiple methods with same name in Python. Function overloading - unlike in Java - isn't supported.

Use default parameters or **kwargs and *args arguments.

You can make static methods or class methods with the @staticmethod or @classmethod decorator to return an instance of your class, or to add other constructors.

I advise you to do:

class F:

    def __init__(self, timestamp=0, data=None, metadata=None):
        self.timestamp = timestamp
        self.data = list() if data is None else data
        self.metadata = dict() if metadata is None else metadata

    @classmethod
    def from_file(cls, path):
       _file = cls.get_file(path)
       timestamp = _file.get_timestamp()
       data = _file.get_data()
       metadata = _file.get_metadata()       
       return cls(timestamp, data, metadata)

    @classmethod
    def from_metadata(cls, timestamp, data, metadata):
        return cls(timestamp, data, metadata)

    @staticmethod
    def get_file(path):
        # ...
        pass

⚠ Never have mutable types as defaults in python. ⚠ See here.