Learning Python: Hour 1

General

Everything(!) is an object

if __name__ == ‘__main__’:

Collections

Dictionaries use squiqqly brackets

Lists use square brackets

Tuples use parens

Dictionaries

Basic syntax is

• foo{‘bar’:'baz’}
• foo['bar']
• del foo['bar'] to delete
• foo.clear() to clear
• foo.keys() returns a list (need the parens)
• foo.values() returns a list

Lists

Basic syntax is

• li = ['a','b','c','d','e']
• li[0]
• li[-1] is OK (gets the last element in the list)

Slicing

• li[1:3] is ['b','c']
• li[1:-1] is ['b','c','d']
  • So this is different that referencing an element just by index. It’s off by one. It means more of a “leave out the last x elements”
• li[0:3] is ['a','b','c']
• li[:-1] is OK
• li[:] returns a copy of li, same values but a different instance

Adding elements

• li.append(value), li.extend(list of values), li.insert(index, value)
• append vs. extend
  • extend joins the lists. append just adds a value to the end of the list

Searching

• li.index(value)
• value in li (‘a’ in li returns True/False)

Removing elements

• li.remove(value)
• li.pop()

List arithmetic

• ['a','b'] * 3 == ['a','b','a','b','a','b']
• ['a','b'] + ['c'] = ['a','b','c']
  • concatenation, can only be done with lists, can’t add a single value, have to add another list

Tuples (immutable lists)

• no methods (append(), remove(), index() are all invalid)
• can’t be modified once defined
• in CAN be used on a tuple
  • ‘a’ in (‘a’,'b’,'c’) is OK (returns True)
• faster (lookup-wise) than lists
• can be used as a dictionary key (lists can’t)
• used in string formatting

Simultaneous multi-value assignment:

• v = (‘a’,'b’,'c’)
• (x,y,z) = v
  • x is ‘a’, y is ‘b’, z is ‘c’

Consecutive value assignment

• (MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY) = range(7)
  • MONDAY is 0, TUESDAY is 1, etc.

String formatting

• print “Hello %s” % (‘world’,)
• Second argument must be a tuple