Mostly Python 2.x, some Python 3.x tips. Some Python 2.x stuff fixed in Python 3.x

Aim: Avoid falling into pitfalls.

Not a lot of dark corners in Python. Not compared with C, C++.

Python is deceptive. Great language for professional programmers.

I love C. Have made lots of money from C.

Whitespace

• Love whitespace, hate tabs.
• Ban tabs. Use Python aware IDE.
• Configure editor to insert 4 spaces when you push tab.
• python -t, warn inconsitant spaces.
• Mixing of tabs is a problem. 3 spaces? 4 spaces? 8 spaces?

References

• names/references not variables
• slices important
• id(name)
• copy list:
  • f = a[:]
  • f = list(a)
• everything is an object
• everything can be referenced.
• references to functions or class names
• str = ‘abc’
  • just redefined str
  • str is not a reserved word
  • import __builtin__
  • str = __builtin__.str
• False , True = True, False
  • print True
  • illegal in Python 3. Is reserved word.
• None = 77
  • Not in 2.7 or above. Is reserved word.

Variables

• easy to create names accidentally
• pylint, pychecker, etc to check code

Name space rules

• local vs global variables
• scope is where variable is declared
• Modify variable rebinds it, possibly to different scope. May hide global variable in local scope before variable rebind.
• global keyword
• Move main line into seperate function, so variables aren’t global.

Funky Syntax

• if you_need_to_save_a_line: don’t do this
• please don’t do this; then this
• a = a.reverse() returns None
• s = “Spam”; s.reverse(); error can’t change strings
• s[::-1] reverses string, odd - not dark
• split and join. “,”.join(list)
• printing to stderr. print » sys.stderr, s

Types

• Object can change their type. No they can’t.
• Use duck typing, not RTTl. If it looks like a duck, sounds like a duck, it is a duck.
• tuppance: (5,)
• floating point: 1.1 + 1.1 + 1.1 - 3.3 not 0. Close. Decimal class.
• ++++++++++n same as ++n same as +n
• n++ error.
• n =+ 1 instead of n +1 1
• n = 0 < x == y clever not good.

Default arguments

• def fs(x=[]): print x
• keeps the old value
• static local variable

The library

• which library am I support to use?
• HTTP?
• sqlite?
• argument parsing?
• Py3k is better.
• Don’t use deprecated modules

Concurrency

• dark corner
• GIL - CPython
• Multicore / multithreaded