Python: Programming#
# Single line comments start with a number symbol.
"""
Multiline strings can be written
using three "s, and are often used
as documentation.
"""
Primitive Datatypes and Operators#
You have numbers
3
Math is what you would expect
1 + 1 # => 2
8 - 1 # => 7
10 * 2 # => 20
35 / 5 # => 7.0
Integer division rounds down for both positive and negative numbers.
5 // 3 # => 1
-5 // 3 # => -2
5.0 // 3.0 # => 1.0 # works on floats too
-5.0 // 3.0 # => -2.0
The result of division is always a float
10.0 / 3 # => 3.3333333333333335
Modulo operation
7 % 3 # => 1
i % j have the same sign as j, unlike C
-7 % 3 # => 2
Exponentiation (x**y, x to the yth power)
2**3 # => 8
Enforce precedence with parentheses
1 + 3 * 2 # => 7
(1 + 3) * 2 # => 8
Boolean values are primitives (Note: the capitalization)
True # => True
False # => False
Negate with not
not True # => False
not False # => True
Boolean Operators: Note “and” and “or” are case-sensitive
True and False # => False
False or True # => True
True and False are actually 1 and 0 but with different keywords
True + True # => 2
True * 8 # => 8
False - 5 # => -5
Comparison operators look at the numerical value of True and False
0 == False # => True
2 > True # => True
2 == True # => False
-5 != False # => True
None, 0, and empty strings/lists/dicts/tuples/sets all evaluate to False. All other values are True
bool(0) # => False
bool("") # => False
bool([]) # => False
bool({}) # => False
bool(()) # => False
bool(set()) # => False
bool(4) # => True
bool(-6) # => True
Using boolean logical operators on ints casts them to booleans for evaluation, but their non-cast value is returned. Don’t mix up with bool(ints) and bitwise and/or (&,|)
bool(0) # => False
bool(2) # => True
0 and 2 # => 0
bool(-5) # => True
bool(2) # => True
-5 or 0 # => -5
Equality is ==
1 == 1 # => True
2 == 1 # => False
Inequality is !=
1 != 1 # => False
2 != 1 # => True
More comparisons
1 < 10 # => True
1 > 10 # => False
2 <= 2 # => True
2 >= 2 # => True
Seeing whether a value is in a range
1 < 2 and 2 < 3 # => True
2 < 3 and 3 < 2 # => False
Chaining makes this look nicer
1 < 2 < 3 # => True
2 < 3 < 2 # => False
(is vs. ==) is checks if two variables refer to the same object, but == checks if the objects pointed to have the same values.
a = [1, 2, 3, 4] # Point a at a new list, [1, 2, 3, 4]
b = a # Point b at what a is pointing to
b is a # => True, a and b refer to the same object
b == a # => True, a's and b's objects are equal
b = [1, 2, 3, 4] # Point b at a new list, [1, 2, 3, 4]
b is a # => False, a and b do not refer to the same object
b == a # => True, a's and b's objects are equal
Strings are created with ” or ’
"This is a string."
'This is also a string.'
Strings can be added too
"Hello " + "world!" # => "Hello world!"
String literals (but not variables) can be concatenated without using ‘+’
"Hello " "world!" # => "Hello world!"
A string can be treated like a list of characters
"Hello world!"[0] # => 'H'
You can find the length of a string
len("This is a string") # => 16
Since Python 3.6, you can use f-strings or formatted string literals.
name = "Reiko"
f"She said her name is {name}." # => "She said her name is Reiko"
Any valid Python expression inside these braces is returned to the string.
f"{name} is {len(name)} characters long." # => "Reiko is 5 characters long."
None is an object
None # => None
Don’t use the equality “==” symbol to compare objects to None. Use “is” instead. This checks for equality of object identity.
"etc" is None # => False
None is None # => True
2. Variables and Collections#
Python has a print function
print("I'm Python. Nice to meet you!") # => I'm Python. Nice to meet you!
By default the print function also prints out a newline at the end. Use the optional argument end to change the end string.
print("Hello, World", end="!") # => Hello, World!
Simple way to get input data from console
input_string_var = input("Enter some data: ") # Returns the data as a string
There are no declarations, only assignments. Convention is to use lower_case_with_underscores
some_var = 5
some_var # => 5
Accessing a previously unassigned variable is an exception. See Control Flow to learn more about exception handling.
some_unknown_var # Raises a NameError
If can be used as an expression. Equivalent of C’s ‘?:’ ternary operator
"yay!" if 0 > 1 else "nay!" # => "nay!"
Lists store sequences
li = []
You can start with a prefilled list
other_li = [4, 5, 6]
Add stuff to the end of a list with append
li.append(1) # li is now [1]
li.append(2) # li is now [1, 2]
li.append(4) # li is now [1, 2, 4]
li.append(3) # li is now [1, 2, 4, 3]
Remove from the end with pop
li.pop() # => 3 and li is now [1, 2, 4]
Let’s put it back
li.append(3) # li is now [1, 2, 4, 3] again.
Access a list like you would any array
li[0] # => 1
Look at the last element
li[-1] # => 3
Looking out of bounds is an IndexError
li[4] # Raises an IndexError
You can look at ranges with slice syntax. The start index is included, the end index is not (It’s a closed/open range for you mathy types.)
li[1:3] # Return list from index 1 to 3 => [2, 4]
li[2:] # Return list starting from index 2 => [4, 3]
li[:3] # Return list from beginning until index 3 => [1, 2, 4]
li[::2] # Return list selecting every second entry => [1, 4]
li[::-1] # Return list in reverse order => [3, 4, 2, 1]
# Use any combination of these to make advanced slices
# li[start:end:step]
Make a one layer deep copy using slices
li2 = li[:] # => li2 = [1, 2, 4, 3] but (li2 is li) will result in false.
Remove arbitrary elements from a list with “del”
del li[2] # li is now [1, 2, 3]
Remove first occurrence of a value
li.remove(2) # li is now [1, 3]
li.remove(2) # Raises a ValueError as 2 is not in the list
Insert an element at a specific index
li.insert(1, 2) # li is now [1, 2, 3] again
Get the index of the first item found matching the argument
li.index(2) # => 1
li.index(4) # Raises a ValueError as 4 is not in the list
You can add lists. Note: values for li and for other_li are not modified.
li + other_li # => [1, 2, 3, 4, 5, 6]
Concatenate lists with “extend()”
li.extend(other_li) # Now li is [1, 2, 3, 4, 5, 6]
Check for existence in a list with “in”
1 in li # => True
Examine the length with “len()”
len(li) # => 6
Tuples are like lists but are immutable.
tup = (1, 2, 3)
tup[0] # => 1
tup[0] = 3 # Raises a TypeError
Note that a tuple of length one has to have a comma after the last element but tuples of other lengths, even zero, do not.
type((1)) # => <class 'int'>
type((1,)) # => <class 'tuple'>
type(()) # => <class 'tuple'>
You can do most of the list operations on tuples too
len(tup) # => 3
tup + (4, 5, 6) # => (1, 2, 3, 4, 5, 6)
tup[:2] # => (1, 2)
2 in tup # => True
# You can unpack tuples (or lists) into variables
a, b, c = (1, 2, 3) # a is now 1, b is now 2 and c is now 3
# You can also do extended unpacking
a, *b, c = (1, 2, 3, 4) # a is now 1, b is now [2, 3] and c is now 4
# Tuples are created by default if you leave out the parentheses
d, e, f = 4, 5, 6 # tuple 4, 5, 6 is unpacked into variables d, e and f
# respectively such that d = 4, e = 5 and f = 6
# Now look how easy it is to swap two values
e, d = d, e # d is now 5 and e is now 4
Dictionaries store mappings from keys to values
empty_dict = {}
# Here is a prefilled dictionary
filled_dict = {"one": 1, "two": 2, "three": 3}
Note keys for dictionaries have to be immutable types. This is to ensure that the key can be converted to a constant hash value for quick look-ups. Immutable types include ints, floats, strings, tuples.
invalid_dict = {[1,2,3]: "123"} # => Yield a TypeError: unhashable type: 'list'
valid_dict = {(1,2,3):[1,2,3]} # Values can be of any type, however.
Look up values with []
filled_dict["one"] # => 1
Get all keys as an iterable with “keys()”. We need to wrap the call in list() to turn it into a list. We’ll talk about those later. Note - for Python versions <3.7, dictionary key ordering is not guaranteed. Your results might not match the example below exactly. However, as of Python 3.7, dictionary items maintain the order at which they are inserted into the dictionary.
list(filled_dict.keys()) # => ["three", "two", "one"] in Python <3.7
list(filled_dict.keys()) # => ["one", "two", "three"] in Python 3.7+
Get all values as an iterable with “values()”. Once again we need to wrap it in list() to get it out of the iterable. Note - Same as above regarding key ordering.
list(filled_dict.values()) # => [3, 2, 1] in Python <3.7
list(filled_dict.values()) # => [1, 2, 3] in Python 3.7+
Check for existence of keys in a dictionary with “in”
"one" in filled_dict # => True
1 in filled_dict # => False
Looking up a non-existing key is a KeyError
filled_dict["four"] # KeyError
Use get() method to avoid the KeyError
filled_dict.get("one") # => 1
filled_dict.get("four") # => None
# The get method supports a default argument when the value is missing
filled_dict.get("one", 4) # => 1
filled_dict.get("four", 4) # => 4
setdefault() inserts into a dictionary only if the given key isn’t present
filled_dict.setdefault("five", 5) # filled_dict["five"] is set to 5
filled_dict.setdefault("five", 6) # filled_dict["five"] is still 5
Adding to a dictionary
filled_dict.update({"four":4}) # => {"one": 1, "two": 2, "three": 3, "four": 4}
filled_dict["four"] = 4 # another way to add to dict
Remove keys from a dictionary with del
del filled_dict["one"] # Removes the key "one" from filled dict
Sets store … well sets
empty_set = set()
# Initialize a set with a bunch of values.
some_set = {1, 1, 2, 2, 3, 4} # some_set is now {1, 2, 3, 4}
Similar to keys of a dictionary, elements of a set have to be immutable.
invalid_set = {[1], 1} # => Raises a TypeError: unhashable type: 'list'
valid_set = {(1,), 1}
Add one more item to the set
filled_set = some_set
filled_set.add(5) # filled_set is now {1, 2, 3, 4, 5}
# Sets do not have duplicate elements
filled_set.add(5) # it remains as before {1, 2, 3, 4, 5}
Do set intersection with &
other_set = {3, 4, 5, 6}
filled_set & other_set # => {3, 4, 5}
Do set union with |
filled_set | other_set # => {1, 2, 3, 4, 5, 6}
Do set difference with -
{1, 2, 3, 4} - {2, 3, 5} # => {1, 4}
Do set symmetric difference with ^
{1, 2, 3, 4} ^ {2, 3, 5} # => {1, 4, 5}
Check if set on the left is a superset of set on the right
{1, 2} >= {1, 2, 3} # => False
Check if set on the left is a subset of set on the right
{1, 2} <= {1, 2, 3} # => True
Check for existence in a set with in
2 in filled_set # => True
10 in filled_set # => False
Make a one layer deep copy
filled_set = some_set.copy() # filled_set is {1, 2, 3, 4, 5}
filled_set is some_set # => False
3. Control Flow and Iterables#
Let’s just make a variable
some_var = 5
Here is an if statement. Indentation is significant in Python! Convention is to use four spaces, not tabs. This prints “some_var is smaller than 10”
if some_var > 10:
print("some_var is totally bigger than 10.")
elif some_var < 10: # This elif clause is optional.
print("some_var is smaller than 10.")
else: # This is optional too.
print("some_var is indeed 10.")
For loops iterate over lists
"""prints:
dog is a mammal
cat is a mammal
mouse is a mammal
"""
for animal in ["dog", "cat", "mouse"]:
# You can use format() to interpolate formatted strings
print("{} is a mammal".format(animal))
range(number) returns an iterable of numbers from zero up to (but excluding) the given number
""" prints:
0
1
2
3
"""
for i in range(4):
print(i)
range(lower, upper) returns an iterable of numbers from the lower number to the upper number
"""prints:
4
5
6
7
"""
for i in range(4, 8):
print(i)
range(lower, upper, step) returns an iterable of numbers from the lower number to the upper number, while incrementing by step. If step is not indicated, the default value is 1.
"""prints:
4
6
"""
for i in range(4, 8, 2):
print(i)
Loop over a list to retrieve both the index and the value of each list item:
animals = ["dog", "cat", "mouse"]
for i, value in enumerate(animals):
print(i, value)
While loops go until a condition is no longer met.
x = 0
while x < 4:
print(x)
x += 1 # Shorthand for x = x + 1
Handle exceptions with a try/except block
try:
# Use "raise" to raise an error
raise IndexError("This is an index error")
except IndexError as e:
pass # Refrain from this, provide a recovery (next example).
except (TypeError, NameError):
pass # Multiple exceptions can be processed jointly.
else: # Optional clause to the try/except block. Must follow
# all except blocks.
print("All good!") # Runs only if the code in try raises no exceptions
finally: # Execute under all circumstances
print("We can clean up resources here")
Instead of try/finally to cleanup resources you can use a with statement
with open("myfile.txt") as f:
for line in f:
print(line)
Writing to a file
contents = {"aa": 12, "bb": 21}
with open("myfile1.txt", "w+") as file:
file.write(str(contents)) # writes a string to a file
import json
with open("myfile2.txt", "w+") as file:
file.write(json.dumps(contents)) # writes an object to a file
Reading from a file
with open('myfile1.txt', "r+") as file:
contents = file.read() # reads a string from a file
print(contents)
# print: {"aa": 12, "bb": 21}
with open('myfile2.txt', "r+") as file:
contents = json.load(file) # reads a json object from a file
print(contents)
# print: {"aa": 12, "bb": 21}
Python offers a fundamental abstraction called the Iterable. An iterable is an object that can be treated as a sequence. The object returned by the range function, is an iterable.
filled_dict = {"one": 1, "two": 2, "three": 3}
our_iterable = filled_dict.keys()
print(our_iterable) # => dict_keys(['one', 'two', 'three']). This is an object
# that implements our Iterable interface.
We can loop over it.
for i in our_iterable:
print(i) # Prints one, two, three
However we cannot address elements by index.
our_iterable[1] # Raises a TypeError
An iterable is an object that knows how to create an iterator.
our_iterator = iter(our_iterable)
Our iterator is an object that can remember the state as we traverse through it. We get the next object with “next()”.
next(our_iterator) # => "one"
It maintains state as we iterate.
next(our_iterator) # => "two"
next(our_iterator) # => "three"
After the iterator has returned all of its data, it raises a stop iteration exception
next(our_iterator) # Raises StopIteration
We can also loop over it, in fact, “for” does this implicitly!
our_iterator = iter(our_iterable)
for i in our_iterator:
print(i) # Prints one, two, three
You can grab all the elements of an iterable or iterator by call of list().
list(our_iterable) # => Returns ["one", "two", "three"]
list(our_iterator) # => Returns [] because state is saved
4. Functions#
Use “def” to create new functions
def add(x, y):
print("x is {} and y is {}".format(x, y))
return x + y # Return values with a return statement
Calling functions with parameters
add(5, 6) # => prints out "x is 5 and y is 6" and returns 11
Another way to call functions is with keyword arguments
add(y=6, x=5) # Keyword arguments can arrive in any order.
You can define functions that take a variable number of positional arguments
def varargs(*args):
return args
varargs(1, 2, 3) # => (1, 2, 3)
You can define functions that take a variable number of keyword arguments, as well
def keyword_args(**kwargs):
return kwargs
keyword_args(big="foot", loch="ness") # => {"big": "foot", "loch": "ness"}
You can do both at once, if you like
def all_the_args(*args, **kwargs):
print(args)
print(kwargs)
"""
all_the_args(1, 2, a=3, b=4) prints:
(1, 2)
{"a": 3, "b": 4}
"""
When calling functions, you can do the opposite of args/kwargs!. Use * to expand tuples and use ** to expand kwargs.
args = (1, 2, 3, 4)
kwargs = {"a": 3, "b": 4}
all_the_args(*args) # equivalent: all_the_args(1, 2, 3, 4)
all_the_args(**kwargs) # equivalent: all_the_args(a=3, b=4)
all_the_args(*args, **kwargs) # equivalent: all_the_args(1, 2, 3, 4, a=3, b=4)
Returning multiple values (with tuple assignments)
def swap(x, y):
return y, x # Return multiple values as a tuple without the parenthesis.
# (Note: parenthesis have been excluded but can be included)
x = 1
y = 2
x, y = swap(x, y) # => x = 2, y = 1
# (x, y) = swap(x,y) # Again the use of parenthesis is optional.
global scope
x = 5
def set_x(num):
# local scope begins here
# local var x not the same as global var x
x = num # => 43
print(x) # => 43
def set_global_x(num):
# global indicates that particular var lives in the global scope
global x
print(x) # => 5
x = num # global var x is now set to 6
print(x) # => 6
set_x(43)
set_global_x(6)
"""
prints:
43
5
6
"""
Python has first class functions
def create_adder(x):
def adder(y):
return x + y
return adder
add_10 = create_adder(10)
add_10(3) # => 13
There are also anonymous functions
(lambda x: x > 2)(3) # => True
(lambda x, y: x ** 2 + y ** 2)(2, 1) # => 5
There are built-in higher order functions
list(map(add_10, [1, 2, 3])) # => [11, 12, 13]
list(map(max, [1, 2, 3], [4, 2, 1])) # => [4, 2, 3]
list(filter(lambda x: x > 5, [3, 4, 5, 6, 7])) # => [6, 7]
We can use list comprehensions for nice maps and filters. List comprehension stores the output as a list (which itself may be nested).
[add_10(i) for i in [1, 2, 3]] # => [11, 12, 13]
[x for x in [3, 4, 5, 6, 7] if x > 5] # => [6, 7]
You can construct set and dict comprehensions as well.
{x for x in 'abcddeef' if x not in 'abc'} # => {'d', 'e', 'f'}
{x: x**2 for x in range(5)} # => {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
5. Classes#
We use the “class” statement to create a class
class Human:
# A class attribute. It is shared by all instances of this class
species = "H. sapiens"
# Basic initializer, this is called when this class is instantiated.
# Note that the double leading and trailing underscores denote objects
# or attributes that are used by Python but that live in user-controlled
# namespaces. Methods(or objects or attributes) like: __init__, __str__,
# __repr__ etc. are called special methods (or sometimes called dunder
# methods). You should not invent such names on your own.
def __init__(self, name):
# Assign the argument to the instance's name attribute
self.name = name
# Initialize property
self._age = 0
# An instance method. All methods take "self" as the first argument
def say(self, msg):
print("{name}: {message}".format(name=self.name, message=msg))
# Another instance method
def sing(self):
return 'yo... yo... microphone check... one two... one two...'
# A class method is shared among all instances
# They are called with the calling class as the first argument
@classmethod
def get_species(cls):
return cls.species
# A static method is called without a class or instance reference
@staticmethod
def grunt():
return "*grunt*"
# A property is just like a getter.
# It turns the method age() into a read-only attribute of the same name.
# There's no need to write trivial getters and setters in Python, though.
@property
def age(self):
return self._age
# This allows the property to be set
@age.setter
def age(self, age):
self._age = age
# This allows the property to be deleted
@age.deleter
def age(self):
del self._age
When a Python interpreter reads a source file it executes all its code. This name check makes sure this code block is only executed when this module is the main program.
if __name__ == '__main__':
# Instantiate a class
i = Human(name="Ian")
i.say("hi") # "Ian: hi"
j = Human("Joel")
j.say("hello") # "Joel: hello"
# i and j are instances of type Human; i.e., they are Human objects.
# Call our class method
i.say(i.get_species()) # "Ian: H. sapiens"
# Change the shared attribute
Human.species = "H. neanderthalensis"
i.say(i.get_species()) # => "Ian: H. neanderthalensis"
j.say(j.get_species()) # => "Joel: H. neanderthalensis"
# Call the static method
print(Human.grunt()) # => "*grunt*"
# Static methods can be called by instances too
print(i.grunt()) # => "*grunt*"
# Update the property for this instance
i.age = 42
# Get the property
i.say(i.age) # => "Ian: 42"
j.say(j.age) # => "Joel: 0"
# Delete the property
del i.age
# i.age # => this would raise an AttributeError
6.1 Inheritance#
Inheritance allows new child classes to be defined that inherit methods and variables from their parent class.
Using the Human class defined above as the base or parent class, we can define a child class, Superhero, which inherits the class variables like “species”, “name”, and “age”, as well as methods, like “sing” and “grunt” from the Human class, but can also have its own unique properties.
To take advantage of modularization by file you could place the classes above in their own files, say, human.py
To import functions from other files use the following format from “filename-without-extension” import “function-or-class”
from human import Human
Specify the parent class(es) as parameters to the class definition
class Superhero(Human):
# If the child class should inherit all of the parent's definitions without
# any modifications, you can just use the "pass" keyword (and nothing else)
# but in this case it is commented out to allow for a unique child class:
# pass
# Child classes can override their parents' attributes
species = 'Superhuman'
# Children automatically inherit their parent class's constructor including
# its arguments, but can also define additional arguments or definitions
# and override its methods such as the class constructor.
# This constructor inherits the "name" argument from the "Human" class and
# adds the "superpower" and "movie" arguments:
def __init__(self, name, movie=False,
superpowers=["super strength", "bulletproofing"]):
# add additional class attributes:
self.fictional = True
self.movie = movie
# be aware of mutable default values, since defaults are shared
self.superpowers = superpowers
# The "super" function lets you access the parent class's methods
# that are overridden by the child, in this case, the __init__ method.
# This calls the parent class constructor:
super().__init__(name)
# override the sing method
def sing(self):
return 'Dun, dun, DUN!'
# add an additional instance method
def boast(self):
for power in self.superpowers:
print("I wield the power of {pow}!".format(pow=power))
if __name__ == '__main__':
sup = Superhero(name="Tick")
# Instance type checks
if isinstance(sup, Human):
print('I am human')
if type(sup) is Superhero:
print('I am a superhero')
# Get the Method Resolution search Order used by both getattr() and super()
# This attribute is dynamic and can be updated
print(Superhero.__mro__) # => (<class '__main__.Superhero'>,
# => <class 'human.Human'>, <class 'object'>)
# Calls parent method but uses its own class attribute
print(sup.get_species()) # => Superhuman
# Calls overridden method
print(sup.sing()) # => Dun, dun, DUN!
# Calls method from Human
sup.say('Spoon') # => Tick: Spoon
# Call method that exists only in Superhero
sup.boast() # => I wield the power of super strength!
# => I wield the power of bulletproofing!
# Inherited class attribute
sup.age = 31
print(sup.age) # => 31
# Attribute that only exists within Superhero
print('Am I Oscar eligible? ' + str(sup.movie))
6.2 Multiple Inheritance#
Another class definition, bat.py
class Bat:
species = 'Baty'
def __init__(self, can_fly=True):
self.fly = can_fly
# This class also has a say method
def say(self, msg):
msg = '... ... ...'
return msg
# And its own method as well
def sonar(self):
return '))) ... ((('
if __name__ == '__main__':
b = Bat()
print(b.say('hello'))
print(b.fly)
And yet another class definition that inherits from Superhero and Bat superhero.py
from superhero import Superhero
from bat import Bat
# Define Batman as a child that inherits from both Superhero and Bat
class Batman(Superhero, Bat):
def __init__(self, *args, **kwargs):
# Typically to inherit attributes you have to call super:
# super(Batman, self).__init__(*args, **kwargs)
# However we are dealing with multiple inheritance here, and super()
# only works with the next base class in the MRO list.
# So instead we explicitly call __init__ for all ancestors.
# The use of *args and **kwargs allows for a clean way to pass
# arguments, with each parent "peeling a layer of the onion".
Superhero.__init__(self, 'anonymous', movie=True,
superpowers=['Wealthy'], *args, **kwargs)
Bat.__init__(self, *args, can_fly=False, **kwargs)
# override the value for the name attribute
self.name = 'Sad Affleck'
def sing(self):
return 'nan nan nan nan nan batman!'
if __name__ == '__main__':
sup = Batman()
# Get the Method Resolution search Order used by both getattr() and super().
# This attribute is dynamic and can be updated
print(Batman.__mro__) # => (<class '__main__.Batman'>,
# => <class 'superhero.Superhero'>,
# => <class 'human.Human'>,
# => <class 'bat.Bat'>, <class 'object'>)
# Calls parent method but uses its own class attribute
print(sup.get_species()) # => Superhuman
# Calls overridden method
print(sup.sing()) # => nan nan nan nan nan batman!
# Calls method from Human, because inheritance order matters
sup.say('I agree') # => Sad Affleck: I agree
# Call method that exists only in 2nd ancestor
print(sup.sonar()) # => ))) ... (((
# Inherited class attribute
sup.age = 100
print(sup.age) # => 100
# Inherited attribute from 2nd ancestor whose default value was overridden.
print('Can I fly? ' + str(sup.fly)) # => Can I fly? False
7. Advanced#
Generators help you make lazy code.
def double_numbers(iterable):
for i in iterable:
yield i + i
Generators are memory-efficient because they only load the data needed to process the next value in the iterable. This allows them to perform operations on otherwise prohibitively large value ranges. NOTE: range replaces xrange in Python 3.
for i in double_numbers(range(1, 900000000)): # `range` is a generator.
print(i)
if i >= 30:
break
Just as you can create a list comprehension, you can create generator comprehensions as well.
values = (-x for x in [1,2,3,4,5])
for x in values:
print(x) # prints -1 -2 -3 -4 -5 to console/terminal
You can also cast a generator comprehension directly to a list.
values = (-x for x in [1,2,3,4,5])
gen_to_list = list(values)
print(gen_to_list) # => [-1, -2, -3, -4, -5]
Decorators#
In this example beg wraps say. If say_please is True then it will change the returned message.
from functools import wraps
def beg(target_function):
@wraps(target_function)
def wrapper(*args, **kwargs):
msg, say_please = target_function(*args, **kwargs)
if say_please:
return "{} {}".format(msg, "Please! I am poor :(")
return msg
return wrapper
@beg
def say(say_please=False):
msg = "Can you buy me a beer?"
return msg, say_please
print(say()) # Can you buy me a beer?
print(say(say_please=True)) # Can you buy me a beer? Please! I am poor :(