Python exercises#
Python as a calculator#
You can use the Python as a simple calculator:
1 + 2
4.5 / 3 - (1 + 2) * 3
Use ** to compute the power:
4**5
Python also supports complex numbers:
a = 1 + 2j
b = 3 - 5j
a * b
“Modulo” (%) computes the remainder of integer division
10 % 2
10 % 3
for n in range(10):
print(f"{n} % 3 = {n % 3}")
Division is exact in Python 3, even if both numbers are integers
3 / 2
Integer division can be done with //
and always returns an integer
3 // 2
More advanced mathematical functions can be imported:
from math import log10
log10(100)
help(log10)
from math import log
Some exercises adapted from Practice Python, used under CC-BY-4.0.
Exercise#
starting from either a variable or a command-line, print whether a number is odd or even.
Modify hw.py.
Bonus: accept any number of arguments
For example:
$ python3 hw.py 5
5 is odd!
$ python3 hw.py 123456
123456 is even!
Bonus: show both quotient and remainder when given two integers
$ python3 hw.py 5 3
7 / 3 = 2 remainder 1
import sys
...
Exercise#
Given a list, return all of the even items.
For example, given [1, 2, 3, 4] return [2, 4].
def get_even(input_list):
"""Return even items found in a list"""
even_items = []
...
return even_items
get_even([1, 2, 3, 4])
Exercise#
Given a list, return a dictionary of the counts of even and odd items
For example, given [0, 1, 2, 3, 4] return {“even”: 2, “odd”: 3}.
def get_even_counts(input_list):
"""Return dict of"""
counts = {}
...
return counts
Exercise#
list all the files in a directory
in alphabetical order (bonus: case-insensitive!)
at the end, show counts by file extension
paths = ...
sorted_paths = ...
extension_counts = {}
for path in sorted_paths:
...
Subclassing shapes#
Let’s define a base shape class with methods to calculate area and perimiter:
class Shape:
"""A class defining Shapes"""
def __str__(self):
return f"I am a {self.__class__.__name__}"
def area(self):
raise NotImplementedError()
def perimiter(self):
raise NotImplementedError()
This base class defines an API for shapes. Subclasses can impelement this API by overriding these methods.
Now we can make a subclass, Rectangle, which has a width and height:
class Rectangle(Shape):
"""A rectangle"""
def __init__(self, width, height):
self.width = width
self.height = height
def area(self):
return self.width * self.height
def perimiter(self):
return 2 * (self.width + self.height)
r = Rectangle(3, 4)
print(r)
print(f"area = {r.area()}")
print(f"perimiter = {r.perimiter()}")
I am a Rectangle
area = 12
perimiter = 14
Excercise#
Define a Square shape as simply as you can,
such that the constructor takes only one argument:
sq = Square(5)
sq.area() # 25
sq.perimiter() # 20
and define a Circle class that takes a radius:
circ = Circle(radius=5)
circ.area() #
circ.perimiter()
class Square(...):
...
sq = Square(5)
print(sq)
print(f"area = {sq.area()}")
print(f"perimiter = {sq.perimiter()}")
import ...
class Circle(...):
...
circ = Circle(radius=5)
print(circ)
print(f"area = {circ.area()}")
print(f"perimiter = {circ.perimiter()}")
Overriding operators with magic methods#
__add__ and __mul__ are the methods for addition and multiplication.
Check out the Python data model for what operator methods are called.
class MyNumber:
def __init__(self, n):
self.n = n
def __repr__(self):
return f"MyNumber({self.n})"
def __add__(self, other):
if not isinstance(other, MyNumber):
raise TypeError("Can only add MyNumbers together")
return MyNumber(self.n + other.n)
def __mul__(self, other):
if not isinstance(other, MyNumber):
raise TypeError("Can only multiply MyNumbers together")
return MyNumber(self.n * other.n)
a = MyNumber(5)
b = MyNumber(10)
a + b
a * b
But we can’t add a together with a regular Python int!
a + 1
Exercise#
allow multiplication / addition to accept regular Python floats and ints, in addition to MyNumber classes.
class MyNumber:
def __init__(self, n):
self.n = n
def __repr__(self):
return f"MyNumber({self.n})"
def __add__(self, other):
if not isinstance(other, MyNumber):
raise TypeError("Can only add MyNumbers together")
return MyNumber(self.n + other.n)
def __mul__(self, other):
if not isinstance(other, MyNumber):
raise TypeError("Can only multiply MyNumbers together")
return MyNumber(self.n * other.n)
a = MyNumber(5)
print(a + 10)
print(a * 2)
Exercise#
define a Point class which supports addition and subtraction of other Points and multiplication by a scalar (int or float).
Bonus: implement a 3d Point3d class with x, y, z
Bonus: implement a dot product with the @ matrix multiplication operator:
a @ b -> scalar
Bonus: implement an n-dimensional Point class that works for any number of dimensions:
v = Point(1, -2, 5, 6)
len(v) # 4
u = Point(-1, 2, -5, -6)
u + v # (0, 0, 0, 0)
class Point:
def __init__(self, x, y):
...
def __repr__(self):
return f"Point({self.x}, {self.y})"
def __add__(self, other):
...
def __mul__(self, n):
...
def __...__(self, other):
...