Why python?

“Python is the second best language for everything.”

Python

• A multiuse programming language that provides interfaces to almost anything you’d want to do
• In recent years, development of tools like numpy, pandas, and scikit-learn have made it a formidable choice for data analysis and cleaning
• Many firms use python as their program of choice
• Interpreted language

Python Editors

• Personal preferences:
  • Jupyter Notebooks/Anaconda
    • To open a Jupyter server (from terminal):

    jupyter notebook

    • We (hopefully) have access to a Jupyter instance via UW here.
  • VS Code
    • Nice editor with tons of language-specific extensions (like for git, latex, quarto, ipython, julia, etc.)

Pylint

• Companies generally want code to confirm to their stylistic standards
• There are general best practices for python code formatting
• pylint checks code and ensures that there are no syntax errors or formatting errors
• I will not require linting in this class – but it’s good practice

Scalar Data Types¹

• None: The Python “null” value (only one instance of the None object exists)
• str: String type; holds Unicode strings
• bytes: Raw binary data
• float: Double-precision floating-point number (note there is no separate double type)
• bool: A Boolean True or False value
• int: Arbitrary precision integer

Strings

We can concatenate strings with +

print('Peanut Butter' + 'Jelly')

Peanut ButterJelly

str_list = ['I', 'love', 'UW']
' '.join(str_list)

'I love UW'

month = 'April'
f'The month is {month}.'

'The month is April.'

Testing Types

Sometimes you may want to test the type of a scalar in python. The canonical test for whether a variable is null:

var = 2
var is None

False

type(2) is str

False

isinstance(var, int)

True

Lists

Lists are created in brackets, with elements separated by commas:

my_list = ['a', 'b', 'c']
print(my_list)

['a', 'b', 'c']

my_new_list = [x + '_new' for x in my_list]
my_new_list

['a_new', 'b_new', 'c_new']

Exercise: Typing

Use list comprehension to return a list of the types of the elements in the list

types_list = ['a', 1, 2., False, None]

Solutions: Typing

[type(x) for x in types_list]

[str, int, float, bool, NoneType]

Indexing

If we want the first element of a list:

my_list[0]

'a'

my_list[-1]

'c'

my_list[:2]

['a', 'b']

my_list[2:]

['c']

Dictionaries

Dictionaries are similar to lists, except that individual elements are named. They’re created as key:value pairs within braces, separated by commas.

my_dict = {
    'string': 'a', 
    'int': 1,
    'list': my_list
}
my_dict

{'string': 'a', 'int': 1, 'list': ['a', 'b', 'c']}

my_dict['string']

'a'

Dictionaries

You can access all the keys of a dictionary:

my_dict.keys()

dict_keys(['string', 'int', 'list'])

list_len = len(my_list)
idx_seq = range(list_len)
my_new_dict = {x: my_list[x] for x in idx_seq}
my_new_dict

{0: 'a', 1: 'b', 2: 'c'}

list_len == max(idx_seq)

False

Dictionaries

We can also update elements of a dictionary, though we should be a little careful doing this.

my_dict['string'] = 55
my_dict

{'string': 55, 'int': 1, 'list': ['a', 'b', 'c']}

Tuples

Tuples are arrays of elements, created by separating elements by commas between parentheses. They’re very similar to lists, but we cannot change individual elements (immutability).

my_tuple = ('a','b','c')
my_tuple[1] = 'z'

TypeError: 'tuple' object does not support item assignment

a,b = (1,2)
a+b

3

For Loops

We often want to perform an operation/function/chunk of code on every element of a list. For example, we could have used a for loop instead of list comprehension to create my_new_list:

my_newer_list = []
for list_element in my_list:
    my_newer_list.append(list_element + '_new')
my_newer_list

['a_new', 'b_new', 'c_new']

my_sum = 0
for num in range(1,6):
    my_sum += num
my_sum

15

While Loops

Sometimes we don’t know how many times we want to loop, so a for loop can’t be used. Instead, we want to loop until something happens. In these cases, we can use while loops. For example, we can solve the problem

\[\max_n\left\{\sum_{i=1}^ni \leq 2024\right\}\]

While Loops

my_sum = 0
n = 0
while my_sum <= 2024:
    n += 1
    my_sum += n
n, my_sum

(64, 2080)

my_sum = 0
n = 0
while my_sum + (n+1) <= 2024:
    n += 1
    my_sum += n
n, my_sum

(63, 2016)

if/else

Sometimes we want to have conditional logic in our loops (or in our code more generally)

a_mixed_list = [1, 5, '2', 4, '12', '02']
out = 0
for i in range(len(a_mixed_list)):
    out += a_mixed_list[i]
out

TypeError: unsupported operand type(s) for +=: 'int' and 'str'

for i in range(len(a_mixed_list)):
    if isinstance(a_mixed_list[i], str):
        out += float(a_mixed_list[i])
    else:
        out += a_mixed_list[i]
out

32.0

elif

Before we had an either/or – using elif, we can add multiple possibilities:

data_list = [10, 20, 30, 15, 25]
label_list = ['red', 'green', 'blue', 'red', 'green']
red_sum, green_sum, blue_sum = 0,0,0
for i in range(len(data_list)):
    if label_list[i] == 'red':
        red_sum += data_list[i]
    elif label_list[i]  == 'green':
        green_sum += data_list[i]
    else:
        blue_sum += data_list[i]
red_sum, green_sum, blue_sum

(25, 45, 30)

pass

Python’s “do nothing” statement

x = 0
if x < 0:
    print("negative!")
elif x == 0:
    # TODO: put something smart here
    pass
else:
    print("positive!")

break

We can exit for loops entirely using break – let’s revisit our earlier sum problem:

my_sum = 0
n = 0
for i in range(2024):
    if my_sum > 2024-i:
        n += i-1
        break
    my_sum += i
n, my_sum

(63, 2016)

Exercise: Factorial

Write a loop that takes a natural number \(n>0\) and returns \(n!\), where

\[ n! = \prod_{i=1}^ni \]

If you have time, write a loop that computes the binomial coefficient:

\[ \binom{n}{k} = \frac{n!}{k!(n-k)!} \]

Solution (First Part)

n = 5
n_fac = 1
for i in range(1,n+1):
    n_fac *= i
n_fac

120

Solution (Second Part)

Note that

\[ \frac{n!}{k!(n-k)!} = \frac{\prod_{i=k+1}^ni}{(n-k)!} \]

k = 3
n_minus_k_fac = 1
for i in range(1,n-k+1):
    n_minus_k_fac *= i
n_choose_k = 1
for i in range(k+1, n+1):
    n_choose_k *= i
n_choose_k /= n_minus_k_fac
n_choose_k

10.0

Why?

• In the second solution we reused code that we already wrote for the first part
• If we do this enough, we’ll make mistakes that are hard to track down
• If we want to repeatedly perform the same operation, we should write a function

DRY (Don’t Repeat Yourself)

• If you’re copy/pasting code, you could probably do something differently to avoid that
• If you find a way to avoid doing so, your code will be better
• So: try not to “repeat yourself” in this way

Basic Function Syntax

Functions can be defined very generally in python

def square(x):
    return x**2

square(2)

4

Docstrings

In general, we want other users of our code (or our future selves) to be able to understand what functions do. This is particularly important later when we get to classes.

def square(x):
    """
    This function takes as input a scalar and outputs the scalar squared.
    """
    return x**2

square(2)

4

Function Annotations

Sometimes, it might not be obvious what types of arguments should be passed to functions. For example, our function square cannot take a string as its argument. We can help users out by showing what the input of a function should be and what it will output.

def square(x: float) -> float:
    """
    This function takes as input a scalar and outputs the scalar squared.
    """
    return x**2

square(2)

4

Multiple Arguments

Functions can take more than one argument. For example, suppose we wanted to calculate the value \(y\) corresponding to point \(x\) on the linear function \(y=mx+b\).

def linear_function(x: float, m: float, b: float) -> float:
    """
    Given the slope (m) and intercept (b) of a line, calculate 
    the y coordinate corresponding to x.
    """
    return m*x + b

linear_function(x=3., m=-1., b=0.)

-3.0

Function Defaults

Python makes it very easy to supply default arguments to functions. In the case below, we provide a default slope of \(-1\) and default intercept of \(0\).

def linear_function(x: float, m: float=-1, b: float=0) -> float:
    """
    Given the slope (m) and intercept (b) of a line, calculate 
    the y coordinate corresponding to x.
    """
    return m*x + b

linear_function(3.)

-3.0

datetime Import

datetime has a module by the same name, so normally it looks something like this

from datetime import datetime, date, time, timedelta

pip install datetime

Creating a datetime Object

Passing integers to datetime yields a datetime object

dt = datetime(2011, 10, 29, 20, 30, 21)
dt.day

29

dt.minute

30

Useful Methods

dt.date()

datetime.date(2011, 10, 29)

dt.time()

datetime.time(20, 30, 21)

dt.strftime("%Y-%m-%d %H:%M")

'2011-10-29 20:30'

datetime.strptime("20091031", "%Y%m%d")

datetime.datetime(2009, 10, 31, 0, 0)

Time Differences

dt2 = datetime(2011, 11, 15, 22, 30)
delta = dt2 - dt
delta

datetime.timedelta(days=17, seconds=7179)

dt + delta

datetime.datetime(2011, 11, 15, 22, 30)

Exercise: datetime

Create a list of the date of every day we have class this quarter. You may ignore holidays.

• Hints:
  • The last day of classes is May 31, 2024
  • You can create a timedelta object using timedelta(days=...)

Solutions: datetime

monday = datetime(2024, 3, 25)
wednesday = datetime(2024, 3, 27)
week = timedelta(days=7)
class_dates = []

while wednesday < datetime(2024, 5, 31):
    class_dates.append(monday)
    class_dates.append(wednesday)
    monday += week
    wednesday += week

[x.strftime("%Y-%m-%d") for x in class_dates]

Importing

Module: file with a .py extension containing Python code

# some_module.py
PI = 3.14159

def f(x):
    return x + 2

def g(a, b):
    return a + b

import some_module
result = some_module.f(5)
pi = some_module.PI

Alternative Import

from some_module import g, PI
result = g(5, PI)

from some_module import *

as

import some_module as sm
from some_module import PI as pi, g as gf

r1 = sm.f(pi)
r2 = gf(6, pi)

Version Control

• When you use a program like Microsoft Word or Dropbox, you have version control built in
  • Ability to revert document or file to a previous version
• This is really important for code as well
  • Imagine you make a bunch of edits to some code to improve readability
  • You rerun the code and your results change
  • Without version control, it’ll take a lot of work to figure out what changed!

Collaboration

• You’ll often be working with other teammates on projects
• How do you coordinate editing files to make sure that everyone’s using the most recent version?
• What do you do if two people edit the same line of code?
• git has tools to help you do these things

GitHub

• I highly recommend using git with GitHub
• GitHub is a great place to publicly post your code for potential employers or collaborators
• In addition, you can sign up for the Student Developer Pack for free
  • Gives you access to Copilot (careful!)
  • Allows you to make private repositories
  • Plenty of other goodies

Setting Up a Repository¹

1. Set up a GitHub Account/install git
2. In your profile on GitHub’s website, in the upper right-hand corner of the “Repositories” tab is a green “New” button.
3. Choose a name and description.
4. For this class’s homework, please keep your repositories public.
5. Check the box for adding a README file, and use the Python .gitignore file.

Setting Up a Repository

6. Click on “Code” and copy the HTTPS or SSH link (based on how you set up your profile)
7. In terminal (Mac or Linux) or Git Bash (Windows), run

git clone <the link you copied>

within the folder that will store your code files.

Files in Git

• There are two versions of every file – the version that’s in the remote repository, and your local version of that file (inclusive of your changes)
• Whenever you want to update the file in the remote repository, you have to do the following:
  1. Stage the file for commit
  2. Commit the files (and write a message)
  3. Pull any edits from the remote
  4. Push your edits to the remote

Step 1: Staging Files

• You created a new file (or edited an existing file) newfile.py and want to add it to the remote repository
• Basic syntax:

If we created newfile.py, we would stage it for commit via

git add newfile.py

We can add all files that have changed using .:

git add .

We can check that it’s been staged:

git status

Step 2: Commiting the Files

• Once the files are staged, we want to write a message explaining what edits we made to the files that we’ve staged
• Basic syntax:

git commit -m "I created a new python file for my repository"

Step 3: Pulling Remote Changes

• If there have been any changes to the remote, we want to pull those down and resolve any conflicts that may exist
• Resolving those changes: kind of a pain!

git pull origin main

• Advanced: if you’re using branches, replace main with your branch name:

git pull origin my_branch

Step 4: Push Your Changes

• Finally, push your changes to the remote!

git push origin main

• Advanced: if you’re using branches, replace main with your branch name:

git push origin my_branch

Step 3.5: Conflicts

• If the remote’s changes and your changes overlap, you need to tell Git how to resolve this

 * branch            main       -> FETCH_HEAD
   3f06dd5..81cf6f5  main       -> origin/main
hint: You have divergent branches and need to specify how to reconcile them.
hint: You can do so by running one of the following commands sometime before
hint: your next pull:
hint:
hint:   git config pull.rebase false  # merge
hint:   git config pull.rebase true   # rebase
hint:   git config pull.ff only       # fast-forward only
hint:
hint: You can replace "git config" with "git config --global" to set a default
hint: preference for all repositories. You can also pass --rebase, --no-rebase,
hint: or --ff-only on the command line to override the configured default per
hint: invocation.
fatal: Need to specify how to reconcile divergent branches.

Recommendation

I think it’s generally good to tell Git to merge by default

git config pull.rebase false

Then when we run git pull origin main, we now get:

 * branch            main       -> FETCH_HEAD
Auto-merging my_file.py
CONFLICT (content): Merge conflict in my_file.py
Automatic merge failed; fix conflicts and then commit the result.

Then git status helpfully tells us:

You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   my_file.py

Fixing the Conflict Manually

If we open my_file.py, Git tells us where the merge issue is and what the two different files say (here the remote has print(greetings) and locally we have print('sup'))

<<<<<<< HEAD
print('greetings')
=======
print('sup')
>>>>>>> 81cf6f5ecf31769420f83647af52c870224d4dd2

All we have to do is keep whichever option we want (manually delete what we don’t want)

Other Options

If we know for sure that we want to keep all the local changes, we can run

git pull origin main -X ours

And similarly, if we know for sure that we want to keep all the remote changes, we can run

git pull origin main -X theirs

Git Basics (Summary)

• If you’re not collaborating, git is quite simple.

  • git add filename ¹ will stage a file for commit
  • git commit -m "this is my change" will add a commit message
  • git pull origin branchname will pull the remote changes
  • git push origin branchname will push your changes to the remote (on branch branchname)

Branches

• Another nice feature of Git is the option to use branches
  • If you branch off of the main repository, you’ll get a copy of that code that you can use as a sandbox, and then merge back in if you want to later
• Potential Uses:
  • Collaboration
  • When there’s a stable version of the code that you don’t want to break

Creating a Branch

• To create a new branch off of an existing branch:

git checkout -b <new branch name> <old branch name>

• To move between branches:

git checkout <branch name>

Merging a Branch

• Usually done via a “pull request”
• I recommend doing this on GitHub via their GUI