CS256
Chris Pollett
Sep 18, 2017
a = "Hello" b = 'Good "bye"' c = """ Triple quotes one can go over multiple lines """ d = ''' this one works as well '''
a = "Hello World" b = a[4] # b is 'o' c = a[:5] # c is 'Hello' d = a[6:] # d is 'World' e = a[3:8] # e is "lo Wo"
my_list = ['YO', 1.2, 7, "HI", ["scary nested list", "watch out!"]] my_empty_list = [] # or list()
b = my_list[0] # b is now: 'YO' c = my_list[4][1] # c is now: 'watch out!' d = my_list[1:3] # d is [1.2, 7] e = my_list[2:] # e is [7, 'HI', ['scary nested list', 'watch out!']]
my_list.append("an end") #my_list now #['YO', 1.2, 7, 'HI', ['scary nested list', 'watch out!'], 'an end'] my_list.insert(2, 3) # my_list now #['YO', 1.2, 3, 7, 'HI', ['scary nested list', 'watch out!'], 'an end'] a = [1, 2, 3] + [4, 5] #a is [1, 2, 3, 4, 5]
import sys if(len(sys.argv)) != 2: #notice sys.argv is a list of command-line args and we found its length print "Please supply a filename" raise SystemExit(1) # throw an error and exit f = open(sys.argv[1]) #program name is argv[0] lines = f.readlines() # reads all lines into list one go f.close() #convert inputs to list of ints ivalues = [int(line) for line in lines] # print min and max print "The min is ", min(ivalues) print "The max is", max(ivalues)
import glob path = './*' files = glob.glob(path) for name in files: print name
a = ( 1, "hello", 3) b = ( some, where) c = "6 scared of 7", "as 7 8 9" #notice can omit paren's d = () # 0-tuple e = 'yo', #one tuple f = ('yo',) #same one tuple g = (d,) # g is ((),)
c = (4, 5) a, b = c
my_set = set([3, 9, 2, 6]) another_set = set("goodness") # set of unique chars print another_set # set(['e', 'd', 'g', 'o', 'n', 's']) if 'e' in another_set: print "it's in there"
a = my_set; b = another_set; c = a | b # union of sets c = a & b # intersection of sets c = a - b # difference of sets c = a ^ b # symmetric difference of sets another_set.add('y') # adds a single element to set another_set.update([6,7,8]) # add multiple elements my_set.remove(3) # removes the number 3 from my_set
person = { "name" : "bob", "age" : 27, "sex" : "Male" } empty_dict = {} #an empty dictionary # or use dict()
name= person["name"] person["age"] = 28 person["address"] = "somewhere" #this would add a key-value pair del person["age"] # removes key value associated with 'age'
if "name" in person: name = person["name"] else: name = "no one" #the above conditional can be shortened to: name = person.get("name", "no one")
keys = list(person) #keys is ['address', 'name', 'sex'] #could also do person.keys() #person.values() would get list of values #person.len() gives the number of keys in dictionary
while condition: statement1 statement2 ...
for n in [1,2,3,4,5,6,7,8,9]: print "2 to the %d is %d" % (n, 2**n)
for n in range(1,10): print "2 to the %d is %d" % (n, 2**n) # same as before
a = range(5) # can omit start to get a = 0, 1, 2, 3, 4 b = range(1,8) # b = 1, 2, 3, 4, 5, 6, 7 c = range(0, 13, 2) # c = 0, 2, 4, 6, 8, 10, 12 d = range(7, 2, -1) # d = 7, 6, 5, 4, 3
a = "Get rich quick" for b in a: print b c = ["now", "I", "know"] for d in c: print c person = { "name" : "bob", "age" : 27, "sex" : "Male" } for key in person: print key, person[key] f = open("my_file.txt") for line in f print line
Which of the following is true?
def smaller_value( a, b): if a < b: return a # notice single line if can be same line else: return b
print smaller_value(8, 4)
def reverse_list(list): if list == []: return [] return reverse_list(list[1:]) + [list[0]] print reverse_list([1, 2, 3, 4]) # prints [4, 3, 2, 1] def divide(a,b): q = a // b r = a - q*b return (q, r) quotient, remainder = divide(2373, 16)
def expify(a, b=2): return b**a #if don't give a second argument b will be 2
i = 5 def printi(): i=4 print i printi() # outputs 4 print i #outputs 5 #note without the i=4 assignment would get i=5
def assign_i(): global i i=3 assign_i() print i #now get 3
a = printi a() # prints 4
Consider: def f(x): return x**3 f(10) #returns 1000 g = lambda x: x**3 #notice don't use return with lambda g(10) #returns 1000 # lambda only works if after : have an expression #we can supply a function as arguments to other functions def a(x): if x >= 0: return 1 else: return 0; def threshold (w, x, activation) : l = len(x) inner = 0; for i in range(0, l): inner += w[i] * x[i] return activation(inner) w = [1, 2] x = [1, 0] threshold (w, x, a) # returns 1 # we can also return functions from functions def make_adder (n): return lambda x: x + n f = make_adder(2) g = make_adder(6) print f(42), g(42)
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__delslice__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__sizeof__', '__str__', '__subclasshook__', 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']
class Stack(object): #this says stack inherits from object a_class_variable = 5 # this var behaves like a Java static var def __init__(self): #self = this in Java self.stack = [] #now stack is a field variable of Stack #in general using self.field_var is how we declare and #instantiate a instance variable def push(self, object): #the first argument of any method self.stack.append(object) # is the object itself def pop(self): return self.stack.pop() @property #properties are computed attributes def length(self): return len(self.stack) #(where an attribute is like a field) of a class # Can use dot notation with or without parentheses to invoke #@property is an example of a Python decorator which is a syntactic #which says this function object immediately after its definition should be #passed to the property() function to get additional features
my_instance = Stack() my_instance.push("hello") print my_instance.length print isinstance(my_instance, object) #returns True print issubclass(Stack, object) #returns True #type(my_instance) returns something containing the word Stack ... #etc
class Stack(list): def push(self, object): #could refer to parent by using syntax list.some_method_of_list #or use super(list, self).some_method_of_list self.append(object)
class MyClass: @staticmethod def my_method(): #some code MyClass.my_method() #similar to Java
#load in abstract class module from abc import ABCMeta, abstractmethod, abstractproperty class MyClass: __metaclass__ = ABCMeta # a metaclass is a class object that knows how to # create other class objects. The default metaclass # is type (Python 3, types.ClassType in Python 2). # Here we are assigning ABCMeta # to be used rather than type # In Python 3, write MyClass(metaclass=ABCmeta) @abstractmethod def my_abstract_method(self): pass @abstractproperty def my_abstract_property(self): pass
try: statement_block_0 except SomeError1 as error_name1: statement_block_1 #executed if a SomeError1 occurs except SomeError2 as error_name2: statement_block_2 ... finally: statement_block_n # always gets executed
try: f = open("file.txt", "r") except IOError as e: print e
raise RuntimeError("Something bad just happened")
class MyException(exception): pass #now could use as: raise MyException("Whoa! A MyException occurred!") #more control can be had by overriding __init__ #here we define an exception taking two arguments class MyException2(exception): def __init__(self, errno, msg): self.args = (errno, msg) self.errno = errno self.errmsg = msg raise MyException2(403, "Access Forbidden")
import div #notice not div.py a, b = div.divide(198, 15) #notice function in div.py have to be prefixed with div.
import div as foo #now foo is the prefix
from div import divide #from div import *; would import all functions print divide(198, 15)
def fact(n): "This function computes a factorial" #can use triple quoted strings if(n <= 1): return 1 else: return n * fact(n - 1)
print fact.__doc__
import test help(test.fact)This would go to a screen that prints the documentation string.
pydoc test.factand get the same result as help(test.fact) before