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CS50 Week 6: Python

Introduction to Python.

Written by Eva Dee on (about a 8 minute read).

Syntax permalink

  • print("hello, world")
  • No semicolons!
  • No need to declare variable types
  • you don't even have to declare them in advance with a var or similar keyword
  • file extension is .py
  • to execute: python
  • for importing libraries: from CS50 import get_string
  • no main function!
  • or instead of ||
  • you can use single or double quotes around characters (only single in C)
  • in python everything that is character-based is a string; it doesn't matter which quotes you use as long as you're consistent
  • # this is a comment
  • elif (instead of else if)
  • not instead of !=
  • ternary operator: letter_only = True if input().isalpha() else False 🤯
  • no do while loops
  • there's no i++ or i--
  • print() adds a new line by default (so you don't need to add \n yourself)
  • append() vs push() for lists (they're not arrays)
  • None instead of NULL
from cs50 import get_string

answer = get_string("What's your name? ")
# no need to deal with separate placeholders
print("hello, " + answer)
# OR
print(f"hello, {answer}"
# you can use formatted string with any string function not just printf 💡
  • counter++ doesn't exist in Python (counter += 1)
# no curly braces, no semi-colons, no extra lines
# but yes, indentation!

# pay attention, : instead of {
if x < y:
print("x is less than y")
# instead of else if
elif x > y:
print("x is greater than y")
print("x is equal to y")

Loops permalink

# capitalizing the booleans
while True:
print("hello, world")

i = 0
while i < 3:
print("hello, world")
i += 1

for i in [0, 1, 2]:

# or
for i in range(3):

Data types permalink

  • Loosely typed language - variable type is inferred from its value (as opposed to C that is a strongly-typed language)
  • bool, float, int, str but also
  • range, list (an array that automatically re-sizes itself), tuple, dict, set (a collection of values without duplicates)
  • no double or long

Speller in Python permalink

words = set()

# this is how you define a function in python
def load(dictionary):
file = open(dictionary, "r")
for line in file:
return True

def check(word):
if word.lower() in words:
return True
return False

def size():
return len(words)

def unload():
return True

🤔 Why is Python slower than C?

  • because Python has to do more work for us with general-purpose solutions, like for memory management
  • you are also incurring some overhead by running the Python interpreter, which reads our source code and translates it to code that our CPU can understand, line by line.

More Python Code permalink

Getting user input permalink

answer = input("What's your name? ")
print(f"hello, {answer}")

Check if a string is in a list permalink

# this is pretty neat
if s.lower() in ["y", "yes"]:

No function prototype, but permalink

# you still need to define something before calling it
def main():

def meow(n):
for i in range(n):


Looping over characters of a string permalink

for c in s:
# the second argument overrides the default \n
print(c.upper(), end="")

Working with command-line arguments permalink

from sys import argv

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

Exit errors permalink

# importing the entire library here
import sys

if len(sys.argv) != 2:
print("missing command-line argument")
print(f"hello, {sys.argv[1]}")

Lists permalink

nums = [1, 2, 3, 4]
# insert 5 at position 4
nums.insert(4, 5)

Dictionary permalink

  • aka associative array
  • under the hood, they are implemented like a hash table
from cs50 import get_string

people = {
"Brian": "+1-617-495-1000",
"David": "+1-949-468-2750"

name = get_string("Name: ")
if name in people:
print(f"Number: {people[name]}")

🤔 How do you run python locally?

  • always in a virtual environment (to make sure you are using the right version of Python when working on any specific project)
  • A Virtual Environment in Python is a self-contained directory that holds a Python installation for a particular language version.
mkdir pyworkshop
cd pyworkshop
python3.7 -m venv env
source env/bin/activate
# and to kill it write deactivate

Some more nifty Python functions

  • type for checking out the type of the variable
  • dir for listing all the methods on the data type
  • help
  • pep8 for python style guide