Python学习7_OnlineResources

Thanks for my new friend who is warm-hearted and nice and beautiful.

The following is my notes.

#!/usr/bin/env python
# import modules used here -- sys is a very standard one
import sys

# Gather our code in a main() function
#sys.argv[0] being the program itself, sys.argv[1] the first argument, and so on
#If you know about argc, or the number of arguments,you can simply request this value from #Python with len(sys.argv)
def main():
    print 'Hello there', sys.argv[1]
    # Command line args are in sys.argv[1], sys.argv[2] ...
    # sys.argv[0] is the script name itself and can be ignored


# Standard boilerplate to call the main() function to begin
# the program.
if __name__ == '__main__':
    main()

Running this program from the command line looks like:

$ python hello.py Guido

Hello there Guido

$ ./hello.py Alice  ## without needing 'python' first (Unix)

Hello there Alice

# Defines a "repeat" function that takes 2 arguments.
def repeat(s, exclaim):
    """
    Returns the string 's' repeated 3 times.
    If exclaim is true, add exclamation marks.
    """
    #we hinted that * works faster than +, the reason being that * calculates the size of 
    #the resulting object once whereas with +, that calculation is made each time + is 
    #called.
    result = s + s + s # can also use "s * 3" which is faster (Why?)
    if exclaim:
        result = result + '!!!'
    return result

 

Below are some ways to call help() and dir() from the interpreter:

• help(len) — help string for the built-in len() function; note that it's "len" not "len()", which is a call to the function, which we don't want
• help(sys) — help string for the sys module (must do an import sys first)
• dir(sys) — dir() is like help() but just gives a quick list of its defined symbols, or "attributes"
• help(sys.exit) — help string for the exit() function in the sys module
• help('xyz'.split) — help string for the split() method for string objects. You can call help() with that object itself or an example of that object, plus its attribute. For example, calling help('xyz'.split) is the same as calling help(str.split).
• help(list) — help string for list objects
• dir(list) — displays list object attributes, including its methods
• help(list.append) — help string for the append() method for list objects

 

 

Backslash escapes work the usual way within both single and double quoted literals -- e.g. \n \' \". A double quoted string literal can contain single quotes without any fuss (e.g. "I didn't do it") and likewise single quoted string can contain double quotes. A string literal can span multiple lines, but there must be a backslash \ at the end of each line to escape the newline.

 

Python strings are "immutable" which means they cannot be changed after they are created (Java strings also use this immutable style). Since strings can't be changed, we construct *new* strings as we go to represent computed values. So for example the expression ('hello' + 'there') takes in the 2 strings 'hello' and 'there' and builds a new string 'hellothere'.

 

Python newbie gotcha: don't use "len" as a variable name to avoid blocking out the len() function. The '+' operator can concatenate two strings. 

 s = 'hi'
  print s[1]          ## i
  print len(s)        ## 2
  print s + ' there'  ## hi there

Unlike Java, the '+' does not automatically convert numbers or other types to string form. The str() function converts values to a string form so they can be combined with other strings.

  pi = 3.14
  ##text = 'The value of pi is ' + pi      ## NO, does not work
  text = 'The value of pi is '  + str(pi)  ## yes

There is no ++ operator, but +=, -=, etc. work. 

 If you want integer division, it is most correct to use 2 slashes -- e.g. 6 // 5 is 1 (previous to python 3000, a single / does int division with ints anyway, but moving forward // is the preferred way to indicate that you want int division.)

 

multi = """It was the best of times.
  It was the worst of times."""

 

Here are some of the most common string methods:

• s.lower(), s.upper() -- returns the lowercase or uppercase version of the string
• s.strip() -- returns a string with whitespace removed from the start and end
• s.isalpha()/s.isdigit()/s.isspace()... -- tests if all the string chars are in the various character classes
• s.startswith('other'), s.endswith('other') -- tests if the string starts or ends with the given other string
• s.find('other') -- searches for the given other string (not a regular expression) within s, and returns the first index where it begins or -1 if not found
• s.replace('old', 'new') -- returns a string where all occurrences of 'old' have been replaced by 'new'
• s.split('delim') -- returns a list of substrings separated by the given delimiter. The delimiter is not a regular expression, it's just text. 'aaa,bbb,ccc'.split(',') -> ['aaa', 'bbb', 'ccc']. As a convenient special case s.split() (with no arguments) splits on all whitespace chars.
• s.join(list) -- opposite of split(), joins the elements in the given list together using the string as the delimiter. e.g. '---'.join(['aaa', 'bbb', 'ccc']) -> aaa---bbb---ccc

• s[1:4] is 'ell' -- chars starting at index 1 and extending up to but not including index 4
• s[1:] is 'ello' -- omitting either index defaults to the start or end of the string
• s[:] is 'Hello' -- omitting both always gives us a copy of the whole thing (this is the pythonic way to copy a sequence like a string or list)
• s[1:100] is 'ello' -- an index that is too big is truncated down to the string length

• s[-1] is 'o' -- last char (1st from the end)
• s[-4] is 'e' -- 4th from the end
• s[:-3] is 'He' -- going up to but not including the last 3 chars.
• s[-3:] is 'llo' -- starting with the 3rd char from the end and extending to the end of the string.

It is a neat truism of slices that for any index n, s[:n] + s[n:] == s.

 

Python has a printf()-like facility to put together a string. The % operator takes a printf-type format string on the left (%d int, %s string, %f/%g floating point), and the matching values in a tuple on the right (a tuple is made of values separated by commas, typically grouped inside parentheses):

# % operator
  text = "%d little pigs come out or I'll %s and %s and %s" % (3, 'huff', 'puff', 'blow down')

The above line is kind of long -- suppose you want to break it into separate lines. You cannot just split the line after the '%' as you might in other languages, since by default Python treats each line as a separate statement (on the plus side, this is why we don't need to type semi-colons on each line). To fix this, enclose the whole expression in an outer set of parenthesis -- then the expression is allowed to span multiple lines. This code-across-lines technique works with the various grouping constructs detailed below: ( ), [ ], { }.

  # add parens to make the long-line work:
  text = ("%d little pigs come out or I'll %s and %s and %s" %
    (3, 'huff', 'puff', 'blow down'))

 

Regular Python strings are *not* unicode, they are just plain bytes. To create a unicode string, use the 'u' prefix on the string literal:

> ustring = u'A unicode \u018e string \xf1'
> ustring
u'A unicode \u018e string \xf1'

A unicode string is a different type of object from regular "str" string, but the unicode string is compatible (they share the common superclass "basestring"), and the various libraries such as regular expressions work correctly if passed a unicode string instead of a regular string.

To convert a unicode string to bytes with an encoding such as 'utf-8', call the ustring.encode('utf-8') method on the unicode string. Going the other direction, the unicode(s, encoding) function converts encoded plain bytes to a unicode string:

## (ustring from above contains a unicode string)
> s = ustring.encode('utf-8')
> s
'A unicode \xc6\x8e string \xc3\xb1'  ## bytes of utf-8 encoding
> t = unicode(s, 'utf-8')             ## Convert bytes back to a unicode string
> t == ustring                      ## It's the same as the original, yay!

True

The built-in print does not work fully with unicode strings. You can encode() first to print in utf-8 or whatever.

 

The "zero" values all count as false: None, 0, empty string, empty list, empty dictionary. 

 

三目运算符in python

return s[:2] + s[(len(s)-2):] if len(s) >= 2 else ''