Introduction to Python

Charles Daniels

September 17, 2020

Introduction

Who Am I?

Why Learn Python?

How Do I Use Python?

I use Python all the time in my work as a graduate student…

Python Essentials

The Basics

Want to Follow Along?

Syntax - Variables

Python syntax is different from C and Java…

# assign a new variable x
x = 7
y = 3
# print x^y
print("{} to the power of {}={}".format(x, y, x**y))

Output:

7 to the power of 3=343

Syntax – Loops

x = 2
while x > 0:
  print("x is ", x)
  x -= 1

for y in range(0, 3):
  print("y is ", y)

Output:

x is 2

x is 1

y is 0

y is 1

y is 2

Syntax – Functions

Defining a function…

def doubleit(x):
  return x * 2

# here, message has a default value
def sayit(x, message="value is: "):
  print(message, x)

print("doubleit(3)=", doubleit(3))
sayit(5)
sayit(5, "different message!")

Output:

doubleit(3)= 6

value is: 5

different message! 5

Syntax – Classes 1

Defining a class…

class Dog:
  # __init__ is the constructor, the first argument
  # doesn't *have* to be "this", this is just a
  # convention ("self" is also popular)
  #
  # __init__ is defined like any other function, this
  # time we use default values
  def __init__(this, fleas=5, greeting="bark"):
    this.fleas = fleas
    this.greeting = greeting

  def bark(this):
    print(this.greeting)

Syntax – Classes 2

Using our class…

fido = Dog()
# single quotes are also allowed for strings
spot = Dog(3, 'woof')
doge = Dog(greeting="wow, such class, very types")

# create a list with the dogs in it
dogs = [fido, spot, doge]

# loop over it
for dog in dogs:
  dog.bark()

Output:

bark

woof

wow, such class, very types

Imports

Some functions, such as sin () are in modules which we must import before we can use them. sin () lives in the math module.

import math

print("pi = ", math.pi)
print("sin(1.5*pi) = ", math.sin(1.5*math.pi))

# we can also import specific items from a module
from math import sin
print("sin(2.5*pi) = ", sin(2.5*math.pi))

Output:

pi = 3.141592653589793

sin(1.5*pi) = -1.0

sin(2.5*pi) = 1.0

Duck Typing

class Duck:
  def quack(this):
    print("Quack quack!")

class Goose:
  def quack(this):
    print("Hong honk!")

duck = Duck()
goose = Goose()
for bird in [duck, goose]:
  bird.quack()

Output:

Quack quack!

Hong honk!

Input – File


# open example.txt for reading, the "with" will
# cause the file to be closed automatically when we
# reach the end of the "with" block, so we don't
# have to call f.close()
with open("example.txt", "r") as f:
  lineno = 0
  for line in f:
    print("line", lineno, "is", line)
    lineno += 1

Output:

line 0 is line 1

line 1 is line two

line 2 is this is the third line

Input – Standard In

This example shows how to loop over all the lines of standard input…

import sys

lineno = 0
for line in sys.stdin:
  print("line", lineno, "is", line)
  lineno += 1

Output – File

with open("output.txt", "w") as f:
  for i in range(5):
    f.write("line #{}\n".format(i))
with open("output.txt", "r") as f:
  for line in f:
    print(line)

Output:

line #0

line #1

line #2

line #3

line #4

Getting Fancy

List Comprehensions (Map)

For a list L, apply a function f to each item, creating a new list L′ such that L′[i] = f(L[i])∀i.

numbers = [1, 2, 3, 4]
squared = [x*x for x in numbers]
print("squared=", squared)

# convert a string to a list of it's ASCII codes
s = "Hello!"
print("characters=", [ord(c) for c in s])

Output:

squared= [1, 4, 9, 16]

characters= [72, 101, 108, 108, 111, 33]

Setting up to Plot

Code taken from matplotlib.org.

import matplotlib
import matplotlib.pyplot as plt
import numpy as np


# Data for plotting
t = np.arange(0.0, 2.0, 0.01)
s = 1 + np.sin(2 * np.pi * t)

Plot the Data

fig, ax = plt.subplots()

ax.plot(t, s)
ax.set(xlabel='time (s)', ylabel='voltage (mV)',
       title='About as simple as it gets, folks')
ax.grid()

# un-comment to save out to a file
# fig.savefig("test.png")

# un-comment to show GUI plot window
# plt.show()

The Result…

Attributes Aren’t Pre-Declared

Remember our class Dog from earlier? This technique is great for annotating objects you didn’t instantiate (but be careful to avoid name collisions)

fido = Dog()
fido.name = "Fido"
fido.bark()
print(fido.name)

Output:

bark

Fido

What Next?

Libraries

How to Install Them

Questions?

End.

Thanks