Python goof ups of a C/C++ programmer

Python is a new age language when compared to good old C. Writing code in Python needs a subtle shift from C mindset. Python offers so many things ready-made that make you feel that you wrote very less code. Anyway, I goofed up while using a very common feature of C: pass by reference.

Python too offers it, but with a caveat: you should know mutable and immutable data objects. When you pass a mutable object like list, dictionary; they can be changed. But, if you pass an immutable data object like string, tuple; they are kept unchanged in caller’s scope. This is very similar to passing “references” (C++) or constant pointers.

  1 class Node:
  2     def __init__(self, value, data=[]):
  3         self.char = value
  4         # We intend to keep a list of values for a key
  5         self.data = data
  6         # XXX: List of Node
  7         self.children = []
  8
  9
 10 n = Node('a')
 11 m = Node('b')
 12
 13 (n.data).append("north")
 14 (m.data).append("south")
 15
 16 print n.data
 17 print m.data

The output of the above code is:

ubuntu@ubuntu:~/Desktop/ToKeep/cprogs/pycon$ python pybug.py
['north', 'south']
['north', 'south']

I don’t quite understand why Python keeps a single instance of default argument(list). Nonetheless, it is interesting.

This thread on Stackoverflow is very informational.

• Omitting “this” pointer

In C++, “this” which is pointer to the object is passed implicitly. Python has similar keyword “self” for this. But, contrary to an implicit understanding, “self” should be an argument to a function, else you would see an error:

NameError: global name ‘self’ is not defined

  1 class pydbg:
  2     def __init__(self, modname=None):
  3         self.level = 0
  4         self.modname = modname
  5
  6     def DLOG(self, lvl=None, fmt=None, *args):
  7         self.level = lvl
  8         print '%s' %self.modname
  9         print fmt
 10         for arg in args:
 11             print arg

So, always add the “self” argument while adding a function in your class.