Lecture 6

Last Time

• We learned some basics about the internet, and technologies like:
• TCP/IP, protocols by which computers can send each other messages across a network of many computers, using IP addresses and port numbers.
• HTTP, a protocol by which browsers, and other programs, can make a request for a webpage (or other content) from a server.
• URLs, including a domain name and parameters like ?q=cats, to pass along additional inputs to a server.
• HTTP status codes, like 404 Not Found, which shows us an error page, and 301 Moved Permanently, which redirects us to the right URL if a website has moved.
• HTML and CSS, languages by which we can format and stylize webpages.
• JavaScript and the DOM, Document Object Model, by which we can change nodes in a tree representation of an HTML page, thereby changing the page itself.

Python

• Python is another programming language, but it is interpreted (run top to bottom by an interpreter, like JavaScript) and higher-level (including features and libraries that are more powerful).
• For example, we can implement the entire resize program in just a few lines with Python:
import sys
from PIL import Image

if len(sys.argv) != 4:
sys.exit("Usage: python resize.py n infile outfile")

n = int(sys.argv[1])
infile = sys.argv[2]
outfile = sys.argv[3]

inimage = Image.open(infile)
width, height = inimage.size
outimage = inimage.resize((width * n, height * n))

outimage.save(outfile)

• First, we import (like include) a sys library (for command-line arguments) and an Image library.
• We check that there are the right number of command-line arguments with len(sys.argv), and then create some variables n, infile, and outfile, without having to specify their types.
• Then, we use the Image library to open the input image, getting its width and height, resizing it with a resize function, and finally saving it to an output file.
• Let’s take a look at some new syntax. In Python, we can create variables with just counter = 0. To increment a variable, we can use counter = counter + 1 or counter += 1.
• Conditions look like:
if x < y:
something
elif:
something
else:
something

• Unlike in C and JavaScript (whereby braces { } are used for blocks of code), the exact indentation of each line is what determines the level of nesting in Python.
• Boolean expressions are slightly different, too:
while True:
something

• Loops can be created with another function, range, that, in the example below, returns a range of numbers from 0, up to but not including 50:
for i in range(50):
something

• In Python, we’ll start by looking at just a few data types:
• bool, True or False
• float, real numbers
• int, integers
• str, strings
• dict, a dictionary of key-value pairs, that act like hash tables
• list, like arrays, but can automatically resize
• range, range of values
• set, a collection of unique things
• tuple, a group of two or more things
• In Python, we can too include the CS50 library, but our syntax will be:
from cs50 import get_float, get_int, get_string

• Notice that we specify the functions we want to use.
• In Python, we can run our program without compiling it with python hello.py (or whatever the name of our file is).
• python is name of the program that we’re actually running at the command line, and it is an interpreter which can read our source code (written in the language Python) and run it, one line at a time. (Technically, there is a compiler that turns our source code into something called bytecode that the interpreter actually runs, but that is abstracted away for us.)

Data types in Python

• Our first hello.py program is just:
print("hello, world")

• Notice that we didn’t need a main function, or anything that we needed to import for the print function. The print function in Python also adds a new line for us automatically.
• Now we can run it with python hello.py.
• We can get strings from a user:
from cs50 import get_string

s = get_string("Name: ")
print("hello,", s)

• We create a variable called s, without specifying the type, and we can pass in multiple variables into the print function, which will print them for us on the same line, separated by a space automatically.
• To avoid the extra spaces, we can put variables inside a string similar to how they are included in C: print(f"hello, {s}"). Here, we’re saying that the string hello, {s} is a formatted string, with the f in front of the string, and so the variable s will be substituted in the string. And we don’t need to worry about the variable type; we can just include them inside strings.
• We can do some math, too:
from cs50 import get_int

x = get_int("x: ")

y = get_int("y: ")

print(f"x + y = {x + y}")
print(f"x - y = {x - y}")
print(f"x * y = {x * y}")
print(f"x / y = {x / y}")
print(f"x mod y = {x % y}")

• Notice that expressions like {x + y} will be evaluated, or calculated, before it’s substituted into the string to be printed.
• By running this program, we see that everything works as we might expect, even dividing two integers to get a floating-point value. (To keep the old behavior of always returning a truncated integer with division, there is the // operator.)
• We can experiment with floating-point values:
from cs50 import get_float

x = get_float("x: ")

y = get_float("y: ")

z = x / y

print(f"x / y = {z}")

• We see the following when we run this program:
\$ python floats.py
x: 1
y: 10
x / y = 0.1

• We can print more decimal places with syntax like print(f"x / y = {z:.50f}"):
x / y = 0.10000000000000000555111512312578270211815834045410

• It turns out that Python still has floating-point imprecision by default, but there are some libraries that will use more memory to store decimal values more precisely.
• We can see if Python has integer overflow:
from time import sleep

i = 1
while True:
print(i)
i *= 2
sleep(1)

• We use the sleep function to pause our program for one second, but double i over and over. And it turns out that integers in Python can be as big as memory allows, so we won’t experience overflow for a much longer time.

Programming in Python

• Let’s take a closer look at conditions:
from cs50 import get_int

# Get x from user
x = get_int("x: ")

# Get y from user
y = get_int("y: ")

# Compare x and y
if x < y:
print("x is less than y")
elif x > y:
print("x is greater than y")
else:
print("x is equal to y")

• Notice that we use consistent indentation, but we don’t need parentheses or braces for our conditions.
• Comments, too, start with just a single # character.
• We can compare strings the way we might expect:
from cs50 import get_char

if c == "Y" or c == "y":
print("yes")
elif c == "N" or c == "n":
print("no")

• Strings can be compared directly, and Boolean expressions can include the words and and or.
• We can write functions in Pythons like this:
def main():
for i in range(3):
cough()

def cough():
"""Cough once"""
print("cough")

if __name__ == "__main__":
main()

• We use the def keyword to define a function cough, indicating that it takes no parameters, or inputs, by using just (), and call it from our main function. Notice that all the code for each function is indented additionally, instead of surrounded by braces.
• Then, at the below, we use a special line if __name__ == "__main__": to call our main function when our program is run. This way, the interpreter will know about the cough function by the time main actually calls it. We could also call cough directly, instead of main, though that would be unconventional in Python. (Instead, we want to try to be “Pythonic”, or following the styles and patterns encouraged by the language and its community.)
• We can add parameters and loops to our cough function, too:
def main():
cough(3)

def cough(n):
for i in range(n):
print("cough")

if __name__ == "__main__":
main()

• n is a variable that can be passed into cough, which we can also pass into range. And notice that we don’t specify types in Python, so n can be of any data type (and can even be assigned to have a value of another type). It’s up to us, the programmer, to use this great power with great responsibility.
• We can define a function to get a positive integer:
from cs50 import get_int

def main():
i = get_positive_int("Positive integer: ")
print(i)

def get_positive_int(prompt):
while True:
n = get_int(prompt)
if n > 0:
break
return n

if __name__ == "__main__":
main()

• Since there is no do-while loop in Python as there is in C, we have a while loop that will go on infinitely, but we use break to end the loop if n > 0. Then, our function will just return n.
• Notice that variables in Python have function scope by default, meaning that n can be initialized within a loop, but still be accessible later in the function.
• We can print each character in a string and capitalize them:
from cs50 import get_string

s = get_string()
for c in s:
print(c.upper(), end="")
print()

• Notice that we can easily iterate over characters in a string with something like for c in s, and we print the uppercase version of each character with c.upper(). Strings in Python are objects, like a data structure with both the value it stores, as well as built-in functions like .upper() that we can call.
• Finally, we pass in another argument to the print function, end="", to prevent a new line from being printed each time. Python has named arguments, where we can name arguments that we can pass in, in addition to positional arguments, based on the position they are in the list. With named arguments, we can pass in arguments in different orders, and omit optional arguments entirely. Notice that this example is labeled with end, indicating the string that we want to end each printed line with. By passing in an empty string, "", nothing will be printed after each character. Before, when we called print without the end argument, the function used \n as the default for end, which is how we got new lines automatically.
• We can get the length of the string with the len() function.
from cs50 import get_string

s = get_string("Name: ")
print(len(s))

• We’ll be using version 3 of Python, which the world is starting to use more and more, so when searching for documentation, we want to be sure that it’s for the right version.
• We can take command-line arguments with:
from sys import argv

if len(argv) == 2:
print(f"hello, {argv[1]}")
else:
print("hello, world")

• We check the number of arguments by looking at the length of argv, a list of arguments, and if there is 2, we print the second one. Like in C, the first command-line argument is the name of the program we wrote, rather than the word python, which is technically the name of the program we run at the command-line.
• We can print each argument in the list:
from sys import argv

for s in argv:
print(s)

• This will iterate over each element in the list argv, allowing us to use it as s.
• And we can iterate over each character, of each argument:
from sys import argv

for s in argv:
for c in s:
print(c)
print()

• We can swap two variables in Python just by reversing their orders:
x = 1
y = 2

print(f"x is {x}, y is {y}")
x, y = y, x
print(f"x is {x}, y is {y}")

• Here, we’re using x, y = y, x to set x to y at the same time as setting y to x.
• We can create a list and add to it:
from cs50 import get_int

numbers = []

# Prompt for numbers (until EOF)
while True:

# Prompt for number
number = get_int("number: ")

# Check for EOF
if not number:
break

# Check whether number is already in list
if number not in numbers:

numbers.append(number)

# Print numbers
print()
for number in numbers:
print(number)

• Here, we create a empty list called numbers with numbers = [], and we get a number from the user. If that number is not already in our list, we add it to our list. We can use not in to check if a value is (not) in a list, and append to add a value to the end of a list.
• We can create our own data structures, objects:
from cs50 import get_string

# Space for students
students = []

# Prompt for students' names and dorms
for i in range(3):
name = get_string("name: ")
dorm = get_string("dorm: ")
students.append({"name": name, "dorm": dorm})

# Print students' names and dorms
for student in students:
print(f"{student['name']} is in {student['dorm']}.")

• We create a list called students, and after we get some input from the user, we append a dictionary of key-value pairs, {"name": name, "dorm": dorm}, to that list. Here, "name" and "dorm" are the keys, and we want their values to be the variables we gathered as input. Then, we can later access each object’s values with student['name'] or student['dorm'] to print them out. In Python, we can index into dictionaries with words or strings, as opposed to just numeric indexes in lists.
• Let’s print four question marks, one at a time:
for i in range(4):
print("?", end="")
print()

• We can print a vertical bar of hash marks, too:
for i in range(3):
print("#")

• And we can print a square with a nested loop:
for i in range(3):
for j in range(3):
print("#", end="")
print()

• Now we can revisit resize.py, and it might make more sense to us now:
from PIL import Image
from sys import argv

if len(sys.argv) != 4:
sys.exit("Usage: python resize.py n infile outfile")

n = int(sys.argv[1])
infile = sys.argv[2]
outfile = sys.argv[3]

inimage = Image.open(infile)
width, height = inimage.size
outimage = inimage.resize((width * n, height * n))

outimage.save(outfile)

• We import the Image library from something called PIL, a free open-source library that we can download and install (which doesn’t come with Python by default).
• Then, we import argv from the system library, and we check our arguments, storing them as n, infile, and outfile, converting the string input for n into an int as we do so.
• By reading the documentation for Python and the Image library, we can open files as an image, getting its size and calling a resize function on it to get another image, which we can then save to another file.
• Let’s look at another example, a spell-checker in Python:
# Words in dictionary
words = set()

def check(word):
"""Return true if word is in dictionary else false"""
return word.lower() in words

"""Load dictionary into memory, returning true if successful else false"""
file = open(dictionary, "r")
for line in file:
file.close()
return True

def size():
"""Returns number of words in dictionary if loaded else 0 if not yet loaded"""
return len(words)


• The functions for dictionary.py are pretty straightforward, since all we need is a set(), a collection into which we can load unique values. In load, we open the dictionary file, and add each line in the file as a word (without the newline character).
• For check, we can just return whether word is in words, and for size, we can just return the length of words. Finally, we don’t need to do anything to unload, since Python manages memory for us.