Notes: Google Python Class

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Day 1

Part 1: Introduction and Strings

  • Common first line of the script: 
    #!/usr/bin/python -tt

(Better: #!/usr/bin/env python)

  • Common start point for stand alone module: 
    if __name__ == '__main__':
        main()
  • String concatenation: 'Hello' + str(6).
  • Comments start with "#".
  • Logic operator: "and", "or" and "not".
  • Format a string: 
    'Hi %s, I have %d donuts.' % ('Alice', 42)
  • String slice: starts from m and goes up to, but not include, n. 
    a = 'Hello'
    a[1:3]    # el
    a[1:]    # ello
    a[:3]    # Hel
  • H e l l o
    • 0 1 2 3 4
    • -5 -4 -3 -2 -1
  • Module 'sys'
    • sys.argv -- a list with the first element as the name of the executing file itself
  • Useful built-in functions:
    • len(list), dir(sys) and help(sys)

Part 2: Lists, Sorting, and Tuples

  • By assigning a to b, it does not make a copy. 
    a = [1, 2, 3]
    b = a
    a[0] = 4
    print b[0]    # 4
To create a copy, use "b = a[:]".
  • To check if a value is in the list, use "in" statement: 
    a = [1, 2, 3]
    1 in a    # True
    4 in a    # False
  • "append(object)" to the end and "pop([index])" the last, if not specified, object -- change the list itself
  • "del" removes the reference, but the object itself will be fine. 
    a = [1, 2, 3]
    b = a
    del a
    print b    # [1, 2, 3]
  • sorted(iterable, cmp=None, key=None, reverse=False) --> new sorted list
"key" can be a function which will be used to apply on each item in the list, and "sorted" will sort the list according to the return value of the "key" function. 
    a = [1, 3, 2]
    sorted(a)    # [1, 2, 3]
    sorted(a, reverse=True)    # [3, 2, 1]
    print a    # [1, 3, 2]
 
    l = ['aaa', 'b', 'cc']
    sorted(l, key=len)    # ['b', 'cc', 'aaa']
  • Concatenation: "xxx".join(list); dissolve: string.split(sep) 
    a = ['a', 'b', 'c']
    b = ':'.join(a)    # 'a:b:c'
    b.split(':')    # ['a', 'b', 'c']
  • When "sorted" works on a list of tuple, it compares the first item in each tuple first; if the first items are equal, it goes to the second one.
To sort something by several criteria, one could compose a key function to return a list of tuple.

Part 3: Dicts and Files

  • d['x'] --> error / d.get('x') --> null
  • 'x' in d --> returns True if there is a key named 'x' in the dict
  • d.keys() / d.values() # same random order
  • d.items() returns a list of tuples with key/value pair in each tuple
  • open(filename, 'rU') --> 'U' argument handles the end of line across different platform
  • print something, --> removes the end of line coming with "print"
  • To read lines in one file: 
    for line in f: print line,
 
    lines = f.readlines()    # returns a list of lines
 
    text = f.read()
  • Tips: incremental programming; good variable names pays.

Day 2

Part 1: Regular Expressions

  • re.search(pat, text) --> returns a "match" object
  • if match: match.group() --> ensures not to call the function upon null object
  • Wild card:
    • . (dot) any char
    • \w word char (letter, digit and underscore bar)
    • \d digit
    • \s whitespace; \S non-whitespace
    • + one or more
    • * zero or more
escape character: \ (e.g. "\." represents a real dot)
  • Use [ ] to embrace a set of characters; dot (.) in [ ] represents a real dot.
  • Use ( ) to emphasize the set of characters we really care; then use index to retrieve the corresponding result. 
    m = re.search(r'([\w.]+)@([\w.]+)', 'blah john.k@gmail.com yatta @ ')
    print m.group()    # john.k@gmail.com
    print m.group(1)    # john.k
    print m.group(2)    # gmail.com
  • re.findall(pat, text) --> returns a list of matches
third parameter "flag": IGNORECASE, DOTALL, etc.

Part 2: Utilities: OS and Commands

os module
  • os.listdir(path)
  • os.path.join(...), os.path.abspath(...), os.path.exists(...)
shutil module
  • shutil.copy(source, dest)
commands module
  • commands.getstatusoutput(cmd) --> returns (status, output) 
    cmd = 'ls -l ' + dir
    (status, output) = commands.getstatusoutput(cmd)
    if status:
        sys.stderr.write('There was an error: ' + output)
        sys.exit(1)
    print output

Part 3: Utilities: URLs and HTTP, Exceptions

  • Exception handler: 
    try:
        f = open(filename)
        text = f.read()
        print text
    except IOError:
        print 'IO Error:', filename
  • Reuse self-built modules, e.g. trythis.py: import trythis
  • Triple quotes allows strings to be across multiple lines. 
    """
    This is a comment
    in multiple lines.
    """
urllib module 
    uf = urllib.urlopen('http://www.google.com')
    uf.read()
 
    urllib.urlretrieve('http://***/foo.gif', 'bar.gif')

Part 4: Closing Thoughts

  • Comprehension syntax -- incorporates for statement with if statement: 
    a = ['aa', 'b', 'ccc']
    [    len(s)     for s in a    ]    # [2, 1, 3]
 
    b = [1, 2, 3, 4]
    [    num * num    for num in b    if num > 2    ]    # [9, 16]
With great power comes great responsibility.

See Also

References

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