Python - default value in class method and self

Question

I have the following class and method:

class Basis(object):

 def __init__(self, P = {\'dimension\': 1, \'generation\':1}):

  self.P = P
  self.P[\'angle\'] = n         


        

Answer 1

Python default values for arguments are evaluated when the function is defined, not when the function is called.

You need to do something like:

def foo(self, x, y=None):
    if y is None:
        y = self.defval

This is also the reason for which having a mutable default (e.g. a dictionary) is a dangerous trap in Python; if you change the value that was given by default you're also changing what will be the default in future calls.

Answer 2

First of all, the defaults for keywords arguments in methods can't be mutable objects, like list, dict, etc. Here you can read why.

Secondly, in def make_projection(self, dimension, angle = self.P['angle']) the self variable is still not defined, so you can get self.P only in method's body.

Try this:

class Basis(object):

  def __init__(self, P=None):
    self.P = P or {'dimension': 1, 'generation':1}
    self.P['angle'] = np.pi/4

  def make_projection(self, dimension, angle=None):
    angle = angle if angle is not None else self.P['angle']  # we must compare to None, because angle can be 0.
    return angle*3