Python Note One

• Hexaadecimals and Octals

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>>>0XAF
175
>>>010
8

• Complex number arithmetic

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>>>import cmath
>>>cmath.sqrt(-1)
1j
>>>(1+3j) * (9+4j)
-3+31j

• #!/usr/bin/env python

• Container consists of sequences and mappings

• Python has six built-in types of sequence: lists, tuples, unicode strings, strings, buffer objects, xrange objects

• extracting elements from right to left:

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>>>numbers[8:3:-1]
[9,8,7,6,5]

• convert between list and string

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>>>list('Hello')
['H', 'e', 'l', 'l', 'o']
>>>''.join(['H', 'e', 'l', 'l', 'o'])
'Hello'

• a.extend(b) can be done by a[len(a):] = b

• numbers.insert(3, 'four') can be done in numbers[3:3]=['four']

• pop remove an element (by default the last one) from the list and return it. pop(0) remove the first

• remove remove the first occurrence of a value

• sorted(cmp, key=None, reverse=False)

• Format specifier:

  • %

  • Conversion flags

    • - left alignment
    • + a sign should precede the convered value.
    • `` a space should precede positive numbers
    • 0 zero padded

  • Minimum field width: if * the width will be read from the value tuple

  • . followed by precision

  • Conversion type:

    • eg x hexadecimal
    • '%x' % 42 => 2a

  • Examples:

    • '%10f' % pi => 3.141593
    • '%10.2f' % pi => 3.14
    • '%.2f' %pi => 3.14
    • '%.5s' % 'Ang Gao' => 'Ang G'
    • '%.*s' % (5, 'Ang 'Gao') => 'Ang G'
    • '%010.2f' % pi => 0000003.14
    • '%-10.2f' % pi => 3.14

• String translate: only works with single character

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>>>from string import maketrans
>>>table = maketrans('cs', 'kz')
>>>len(table)
256
>>>str1.translate(table)

optional second argument can be supplied to translate, specifying letters that should be delete: text.translate(table, 'ab')

• String translate in PY3

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>>>str1.translate({ord('t'):ord('x')})

• String formatting with dict: '%('))le)s' % {'title':'Ang'}

• dict methods

  • copy(): shallow copy
  • deep copy: from copy import deepcopy dc = deepcopy(dit)
  • fromkeys: create new dict with the given keys
    • {}.fromkeys(['name', 'age'])
    • dict.fromkeys(['name', 'age'])
    • dict.fromkeys(['name', 'age'], 'unknown')
  • get: return None if key not exists or supply default value
    • names.get('name', 'default')
  • pop(key): get value, remove key-value pair
  • popitem(): pops off an arbitrary item (key, value)
  • setdefault: similar to get retrieve a value associate with a key, sets the value corresponding to the given key if it is not in dict

• In PY3: a,b,*rest = [1,2,3,4]

• chr(ord('a')) = 'a'

• name = raw_input('Name ') or 'unknown'

• assert expression [, msg]

• else clauses in loop:

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for x in range(10):
    if x == 5:
        break
else:
    print 'only called if loop completed without break'

• exec VS eval

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>>>scope = {}
>>> exec 'sqrt = 1' in scope
>>> scope.keys()
['__builtins__', 'sqrt']

eval(raw_input(...)) has same effect as input(..)

• callable(x) in PY3 hasattr(x, '__call__')

• scoping

  • vars()
    • without argument = locals()
    • with argument = object.__dict__
  • globals(): a dict with the global vars

• change global var

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x = 1
def change_global():
    global x
    x = x + 1

• A function that stores its enclosing scope is called a closure. Normally cannot rebind var in outter scope, in PY3 the keyword nonlocal is introduced. It is used in much the same way as global, and lets you assign vars in outter but none global scopes

• map, filter, reduce moved to functools module