Introduction to Python
Documentation
help(round) # In Jupyter, you can also use: ?round
Help on built-in function round in module builtins:
round(number, ndigits=None)
Round a number to a given precision in decimal digits.
The return value is an integer if ndigits is omitted or None. Otherwise
the return value has the same type as the number. ndigits may be negative.
Variables and assignments
age = 20
firstName = 'Jason'
print(firstName, 'is', age, 'years old')
Jason is 20 years old
a = 1; b = 2 # You can use ; to separate multiple commands in one line
a, b = 1, 2 # You can define multiple variables at once
a = b = 10 # You can assign a value to multiple variables at the same time
b = "b is now a string" # Variables can change type
age += 3 # Another syntax for: age = age + 3
print("Jason's age in three years:", age) # Note that we have to use double quotes here
Jason's age in three years: 23
b in the following code?
a = "left"
b = a
a = "right"
Types
print(type(52))
print(type(52.0))
print(type('52'))
<class 'int'>
<class 'float'>
<class 'str'>
Strings
Use square brackets to get a substring:
element = 'helium'
print(element[0])
print(element[0:3])
h
hel
name = 'Paul'
print(name+' Smith') # You can add strings
print(name*10) # You can replicate strings by mutliplying by a number
print(len(name)) # Strings have lengths
Paul Smith
PaulPaulPaulPaulPaulPaulPaulPaulPaulPaul
4
Lists
A list is a collection of multiple values. This means that multiple values are stored within a single object.
T = [27.3, 27.5, 27.7, 27.5, 27.6]
print('temperature:', T)
print('length:', len(T))
temperature: [27.3, 27.5, 27.7, 27.5, 27.6]
length: 5
Python starts indexing at 0:
print('zeroth item of T is', T[0])
print('fourth item of T is', T[4])
zeroth item of T is 27.3
fourth item of T is 27.6
Lists are mutable (they can be modified):
T[0] = 21.3
print('temperature is now:', T)
temperature is now: [21.3, 27.5, 27.7, 27.5, 27.6]
a = [2, 3, 5]
print('a is initially', a)
a.append(7)
a.append(11)
print('a has become', a)
a is initially [2, 3, 5]
a has become [2, 3, 5, 7, 11]
print('a before', a)
del a[4] # Remove 4th element
print('a after', a)
a before [2, 3, 5, 7, 11]
a after [2, 3, 5, 7]
You can initialize an empty list and fill it in:
a = [] # Start with an empty list
a.append('Vancouver')
a.append('Toronto')
a.append('Kelowna')
print(a)
['Vancouver', 'Toronto', 'Kelowna']
Lists can be heterogeneous and nested:
a = [11, 21, 31]
b = ['Mercury', 'Venus', 'Earth']
c = 'hello'
nestedList = [a, b, c]
print(nestedList)
[[11, 21, 31], ['Mercury', 'Venus', 'Earth'], 'hello']
You can search inside a list:
print('Venus' in b)
print('Mars' in b)
b.index('Venus') # Returns the index
True
False
1
And you sort lists alphabetically:
b.sort()
print(b)
['Earth', 'Mercury', 'Venus']
To delete an item from a list:
b.pop(2) # Using its index
print(b)
b.remove('Earth') # Using its value
print(b)
['Earth', 'Mercury']
['Mercury']
Dictionaries
Dictionaries are unordered sets of key/value pairs.
favs = {'mary': 'orange', 'john': 'green', 'eric': 'blue'}
print(favs)
print(favs['john'])
print(favs['mary'])
{'mary': 'orange', 'john': 'green', 'eric': 'blue'}
green
orange
Now let’s see how to add items to a dictionary:
concepts = {}
concepts['list'] = 'An ordered collection of values'
concepts['dictionary'] = 'A collection of key-value pairs'
print(concepts)
{'list': 'An ordered collection of values', 'dictionary': 'A collection of key-value pairs'}
We can modify the values:
concepts['list'] = 'Simple: ' + concepts['list']
concepts['dictionary'] = 'Complex: ' + concepts['dictionary']
print(concepts)
{'list': 'Simple: An ordered collection of values', 'dictionary': 'Complex: A collection of key-value pairs'}
Deleting dictionary pairs:
del concepts['list'] # Removes the key 'list' and its value
print(concepts)
{'dictionary': 'Complex: A collection of key-value pairs'}
Values can be numerical:
grades = {}
grades['mary'] = 5
grades['john'] = 4.5
print(grades)
{'mary': 5, 'john': 4.5}
The keys can also be numerical:
grades[1] = 2
print(grades)
{'mary': 5, 'john': 4.5, 1: 2}
Conditionals
Python implements conditionals via if, elif (short for “else if”) and else. Use an if statement to control whether some block of code is executed or not.
mass = 3.54
if mass > 3.0:
print(mass, 'is large') # The indentation is important!
3.54 is large
Let’s modify the mass:
mass = 2.07
if mass > 3.0:
print (mass, 'is large')
Add an else statement:
mass = 2.07
if mass > 3.0:
print(mass, 'is large')
else:
print(mass, 'is small')
2.07 is small
Add an elif statement:
x = 5
if x > 0:
print(x, 'is positive')
elif x < 0:
print(x, 'is negative')
else:
print(x, 'is zero')
5 is positive
grade = 85
if grade >= 70:
print('grade is C')
elif grade >= 80:
print('grade is B')
elif grade >= 90:
print('grade is A')
grade is C
Loops
For loops
for number in [2, 3, 5]:
print(number) # The indentation is important!
2
3
5
This is equivalent to:
print(2)
print(3)
print(5)
2
3
5
for number in [2, 3, 5]:
print(number)
print(number)
2
3
5
5
The loop variable could be called anything:
for i in 'hello':
print(i)
h
e
l
l
o
Use range to iterate over a sequence of numbers:
for i in range(0, 3):
print(i)
0
1
2
Let’s add numbers 1 to 10:
total = 0
for number in range(10):
total += number + 1
print(total)
55
While loops
x = 2
while x > 1.0:
x /= 1.1
print(x)
1.8181818181818181
1.652892561983471
1.5026296018031553
1.366026910730141
1.2418426461183099
1.1289478601075542
1.026316236461413
0.9330147604194662
Functions
Python comes with many built-in functions. You can also install packages that contain additional functions. But you can also write your own functions:
def greeting():
print('Hello!')
Let’s run our new function:
greeting()
Hello!
def printDate(year, month, day):
joined = str(year) + '-' + str(month) + '-' + str(day)
print(joined)
printDate(1871, 3, 19)
1871-3-19
Packages
When you launch Python, you can access many functions.
But Python also comes with a standard library of several modules. Each module contains additional functions that can be loaded in a session.
math module.
print('pi is', pi)
NameError: name 'pi' is not defined
import math
print('pi is', math.pi)
pi is 3.141592653589793
You can create an alias from the library:
import math as m
print(m.pi)
3.141592653589793
You can also import math’s items directly:
from math import pi, sin
print(sin(pi/6))
0.49999999999999994
help(math) # Help for libraries works just like help for functions