10. Notes on tricky issues in Python

10.1. Declaration of an array of instance

Given a class Person with no parameters, the correct way to declare an array of Person instances is:

[Person() for x in range(3)]

Do not use the following syntax:

3 * [Person()]

Although this syntax is correct, it provides 3 identical instances (that refer to the same instance), which is not what we want.

10.2. Passing arguments

By default, passing arguments in Python is made by value for simple types and by references for objects. You can not pass an object by value (in C++ you can decide to use or not the & symbol).

The following example illustrate the passing argument by value case:

>>> def swap(a, b):
...    (b, a) = (a, b)
>>> a = 1
>>> b = 2
>>> swap(a, b)
>>> a
1

And this example illustrates the passing argument by reference case:

>>> # First, we create a function that change the attribute 'data'
>>> def init_object(o):
...     o.data = 0.
>>> # Second, we create an empty class that will be populated later
>>> class P(): pass
>>> # Let us create an instance and set the attribute data to 100.
>>> o = P()
>>> o.data = 100.
>>> # let us call the function (by reference)
>>> init_object(o)
>>> o.data
0.0

10.3. The underscore character

• As a convention, the underscore character can be used in front of a variable to indicate it is a private variable, which should not be used then.

• If you use the double underscore prefix such as __arg2, Python uses name-mangling to make it a private variable. However, you can still access to it if you want, so this is not a pure private data.

• Finally, the underscore character can also be used to hide a variable. For instance in a loop statement, the variable to loop over can simply be called _, which is a valid pytho variable:

  for _ in (0,1,2):
      # do something
      print(_)
  

• Note that a variable called _ is often used with the gettext.gettext function.