90% of python in 90 minutes

注:本文整理自 http://www.slideshare.net/MattHarrison4/learn-90


Three Python'isms to Remember

●dir ●help ●colon/indent shuffle

一、hello world

print "hello world" from interpreter $ python >>> print "hello world" hello world REPL Read, Eval, Print, Loop $ python >>> 2 + 2  # read, eval 4 # print >>>   # repeat (loop) Many developers keep a REPL handy during programming From script Make file hello.py with print "hello world" Run with: python hello.py (unix) script Make file hello with #!/usr/bin/env python print "hello world" Run with: chmod +x hello ./hello Python 3 hello world printis no longer a statement, but a function print("hello world")


Everything in Python is an object that has: ●an identity(id) ●a value(mutable or immutable) id >>> a = 4 >>> id(a) 6406896 Value ●Mutable:When you alter the item, the  id is still the same. Dictionary, List ●Immutable:String, Integer, Tuple Mutable >>> b =[] >>> id(b) 140675605442000 >>> b.append(3) >>> b [3] >>> id(b) 140675605442000 # SAME! Immutable >>> a = 4 >>> id(a) 6406896 >>> a =a + 1 >>> id(a) 6406872 # DIFFERENT!


a = 4  # Integer b = 5.6  # Float c = "hello"  # String a = "4"  # rebound to String Long >>> import sys >>> sys.maxint 9223372036854775807 >>> sys.maxint + 1 9223372036854775808L


●lowercase ●underscore_between_words ●don't start with numbers See PEP 8 PEP:Python Enhancement Proposal (similar to JSR in Java)


+, -, *, /, **(power), %(modulo) Careful with integer division >>> 3/4 0 >>> 3/4. 0.75 (In Python 3 // is integer division operator)


name = 'matt' with_quote = "I ain't gonna" longer = """This string has multiple lines in it""" String escaping Escape with \ >>> print 'He said, "I\'m sorry"' He said, "I'm sorry" >>> print '''He said, "I'm sorry"''' He said, "I'm sorry" >>> print """He said, "I'm sorry\"""" He said, "I'm sorry"

String formatting c-like >>> "%s %s" %('hello', 'world') 'hello world' PEP 3101 style >>> "{0} {1}".format('hello', 'world') 'hello world' String Methods ●s.endswith(sub) Returns True if endswith sub ●s.find(sub) Returns index of subor -1 ●s.format(*args) Places args in string ●s.index(sub) Returns index of subor exception ●s.join(list) Returns listitems separated by string ●s.strip() Removes whitespace from start/end

七、Methods & dir

dir Lists attributes and methods: >>> dir("a string") ['__add__', '__class__', ..., 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill'] dunder methods dunder(double under) or "special/magic"  methods determine what will happen  when +(__add__) or /(__div__) is  called. help >>> help("a string".startswith) Help on built-in function startswith: startswith(...) S.startswith(prefix[, start[, end]]) -> bool Return True if S starts with the specified prefix, False otherwise. With optional start, test S beginning at that position. With optional end, stop comparing S at that position. prefix can also be a tuple of strings to try.


Comments follow a # No multi-line comments

九、More Types

None Pythonic way of saying NULL. Evaluates to False. c = None booleans a = True b = False sequences ●lists ●tuples ●sets


Hold sequences. How would we find out the attributes & methods of a list? >>> dir([]) ['__add__', '__class__', '__contains__',... ,'__iter__',... '__len__',... , 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove',  'reverse', 'sort'] >>> a =[] >>> a.append(4) >>> a.append('hello') >>> a.append(1) >>> a.sort() # in place >>> print a [1, 4, 'hello'] How would we find out documentation for a method? >>> help([].append) Help on built-in function append: append(...)

L.append(object) -- append object to end List methods ●l.append(x) Insert x at end of list ●l.extend(l2) Add l2 items to list ●l.sort() In place sort ●l.reverse() Reverse list in place ●l.remove(item) Remove first item found ●l.pop() Remove/return item at end of list


Also called hashmap or associative array elsewhere >>> age ={} >>> age['george'] = 10 >>> age['fred'] = 12 >>> age['henry'] = 10 >>> print age['george'] 10 Find out if 'matt' in age >>> 'matt' in age False in statement Uses __contains__dunder method to determine membership. (Or __iter__as fallback) .get >>> print age['charles'] Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 'charles' >>> print age.get('charles', 'Not found') Not found

deleting keys Removing 'charles' from age >>> delage['charles']

del not in dir.   .pop is an alternative


def add_2(num):

""" return 2 more than num """ return num + 2 five =add_2(3) ●def ●function name ●(parameters) ●:+ indent ●optional documentation ●body ●return whitespace Instead of { use a : and indent consistently (4 spaces) invoke python -tt to error out during inconsistent tab/space usage in a file default (named) parameters def add_n(num, n=3):

"""default to adding 3""" return num +n

five =add_n(2) ten =add_n(15, -5) __doc__ Functions have docstrings. Accessible via .__doc__or help >>> def echo(txt):

...   "echo back txt" ...   return txt

>>> help(echo) Help on function echo in module __main__: <BLANKLINE> echo(txt)

echo back txt

<BLANKLINE> naming ●lowercase ●underscore_between_words ●don't start with numbers ●verb See PEP 8


if grade > 90:

print "A"

elif grade > 80:

print "B"

elif grade > 70:

print "C"


print "D" Remember the colon/whitespace!


Comparison Operators Supports (>, >=, <, <=, ==, !=) >>> 5 > 9 False >>> 'matt' != 'fred' True >>> isinstance('matt', basestring) True >>> type("matt") <type 'str'> Boolean Operators and, or, not(for logical), &, |, and ^(for bitwise) >>> x = 5 >>> x < -4 or x > 4 True Boolean note Parens are only required for precedence if (x > 10):

print "Big"

same as if x > 10:

print "Big" Chained comparisons if 3 <x < 5:

print "Four!"

Same as if x > 3 and x < 5:

print "Four!"


for number in [1,2,3,4,5,6]:

print number

for number in range(1, 7):

print number range note Python tends to follow half-open interval ([start,end)) with range and slices. ●end - start = length ●easy to concat ranges w/o overlap Java/C-esque style of object in array, access (BAD): animals =["cat", "dog", "bird"] for index in range(len(animals)):

print index, animals[index] If you need indices, use enumerate animals =["cat", "dog", "bird"] for index, value in enumerate(animals):

print index, value

Can breakout of nearest loop for item insequence:

# process until first negative if item < 0:


# process item

Can continue to skip over items for item in sequence:

if item < 0: continue # process all positive items

Can loop over lists, strings, iterators, dictionaries... sequence like things: my_dict ={ "name": "matt", "cash": 5.45} for key inmy_dict.keys():

# process key

for value inmy_dict.values():

# process value

for key, value inmy_dict.items():

# process items

pass pass is a null operation for i in range(10):

# do nothing 10 times


Hint: Don't modify list or dictionary contents while looping over them


Sequences (lists, tuples, strings, etc) can be sliced to pull out a single item my_pets =["dog", "cat", "bird"] favorite =my_pets[0] bird =my_pets[-1] Negative Indexing Proper way to think of [negative indexing] is to reinterpret a[-X] as a[len(a)-X]

Slices can take an end index, to pull out a list of items my_pets =["dog", "cat", "bird"]  # a list cat_and_dog =my_pets[0:2] cat_and_dog2 =my_pets[:2] cat_and_bird =my_pets[1:3] cat_and_bird2 =my_pets[1:] Slices can take a stride my_pets =["dog", "cat", "bird"]  # a list dog_and_bird =my_pets[0:3:2] zero_three_etc = range(0,10)[::3] Just to beat it in veg = "tomatoe" correct =veg[:-1] tmte =veg[::2] eotamot =veg[::-1]

十八:File IO

File Input Open a file to read from it (old style): fin = open("foo.txt") for line in fin:

# manipulate line

fin.close() File Output Open a file using 'w' to writeto a file: fout = open("bar.txt", "w") fout.write("hello world") fout.close() Always remember to close your files! closing with with implicit close(new 2.5+ style) with open('bar.txt') as fin: for line in fin:

# process line


class Animal(object):

def __init__(self, name):

self.name =name

def talk(self):

print "Generic Animal Sound"

animal =Animal("thing") animal.talk() notes: ●object(base class) (fixed in 3.X) ●dunder init (constructor) ●all methods take self as first parameter Subclassing class Cat(Animal):

def talk(self):

print '%ssays, "Meow!"' %(self.name)

cat =Cat("Groucho") cat.talk() # invoke method class Cheetah(Cat):

"""classes can hav docstrings""" def talk(self):

print "Growl" naming ●CamelCase ●don't start with numbers ●Nouns


Poor mans print works a lot of the time Remember Clean up print statements. If you really need them, use logging or write to sys.stdout pdb import pdb; pdb.set_trace() pdb commands ●h- help ●s- step into ●n- next ●c- continue ●w- where am I (in stack)? ●l- list code around me




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