Instantiating objects in python

Question

I have a python class I want to instantiate and the __init__ definition has a lot of parameters (10+). Is there a clean way to instantiate a class who's __init__ takes a lot of params?

For example:

class A(object):
 def __init__(self, param1, param2, param3,...param13):
 // create an instance of A
my_a = new A(param1="foo", param2="bar", param3="hello"....)

Is there a cleaner way to do this? like passing in a dictionary or something? Or better, is there an expected convention?

• Tyler Eaves: WHY does the init have so many parameters?
• ThiefMaster: I REALLY Hope your params are not really named like this!

Answer 1

Yes, you can use a dict to collect the parameters:

class A(object):
 def __init__(self, param1, param2, param3):
 print param1, param2, param3
params = {'param1': "foo", 'param2': "bar", 'param3': "hello"}
# no 'new' here, just call the class
my_a = A(**params)

See the unpacking argument lists section of the Python tutorial.

Also, // isn't a comment in Python, it's floor division. # is a comment. For multi-line comments, '''You can use triple single quotes''' or """triple double quotes""".

Answer 2

There is no new keyword in Python. You just invoke the class name as you would a function.

You may specify keyword arguments using a dictionary by prefixing the dictionary with **, for example:

options = {
 "param1": "foo",
 "param2": "bar",
 "param3": "baz"
}
my_a = A(**options)

If you're going to be defining all of the values at once, using a dictionary doesn't really give you any advantage over just specifying them directly while using extra whitespace for clairity:

my_a = A(
 param1 = "foo",
 param2 = "bar",
 param3 = "baz"
)

• agf: You shouldn't call it args, the convention in Python is for args to be a list of positional parameters, not a dict.
• agf: In regards to defining them all at once -- if you're going to use the same parameters in multiple places, you only have to have them in your code once if you use a dict.

Answer 3

its not good passing too many arguments to a constructor. but if you want to do this,try:

class A(object):
 def __init__(self, *args,**kwargs):
 """
 provide list of parameters and documentation
 """
 print *args, **kwargs
params = {'param1': "foo", 'param2': "bar", 'param3': "hello"}
a = A(**params)

Answer 4

Generally, having that many parameters is often a sign that a class is too complicated and should be split up.

If that doesn't apply, pass up a dictionary, or a special parameter object.