Python Cheatsheet

Table Of Contents

Pipenv

Config Files

Read more in this source: Link

Built in packages: configparser, json, yaml, toml Third party packages: dotenv

ini file

# example_ini.ini
[APP]
ENVIRONMENT = test
DEBUG = True
# Only accept True or False

[DATABASE]
USERNAME = xiaoxu
PASSWORD = xiaoxu
HOST = 127.0.0.1
PORT = 5432
DB = xiaoxu_database

yaml file

# example_yaml.yaml
APP:
  ENVIRONMENT: test
  DEBUG: True
  # Only accept True or False

DATABASE:
  USERNAME: xiaoxu
  PASSWORD: xiaoxu
  HOST: 127.0.0.1
  PORT: 5432
  DB: xiaoxu_database

toml file

# example_toml.toml
[APP]
ENVIRONMENT = "test"
DEBUG = true
# Only accept True or False

[DATABASE]
USERNAME = "xiaoxu"
PASSWORD = "xiaoxu"
HOST = "127.0.0.1"
PORT = 5432
DB = "xiaoxu_database"

Pandas

Matplot

A beautiful plotting

complex, real, imag = fourier_transform(pulse_signal, t)
fig, (ax1, ax2) = plt.subplots(2, 1)
fig.suptitle('F.T. of pulse signal')
ax1.plot(t, real, color='green', alpha=0.5)
ax1.set_title('real part')
ax2.plot(t, imag)
ax2.set_title('imag part')

for ax in fig.get_axes():
    ax.label_outer()

NumPy

Basics

Create a NumPy array:

a = np.array([1, 2, 3, 4, 5, 6])

The rank of the array is the number of dimensions. The shape of the array is a tuple of integers giving the size of the array along each dimension.

In NumPy, dimensions are called axes. This means that if you have a 2D array that looks like this:

[[0., 0., 0.],
 [1., 1., 1.]]

Your array has 2 axes. The first axis has a length of 2 and the second axis has a length of 3.

How to create a basic array

np.array(), np.zeros(), np.ones(), np.empty(), np.arange(), np.linspace()

np.arange(2, 9, 2)  # array([2, 4, 6, 8])

You can also use np.linspace() to create an array with values that are spaced linearly in a specified interval:

np.linspace(0, 10, num=5)  # array([ 0. ,  2.5,  5. ,  7.5, 10. ])

The default data type is floating point (np.float64), you can explicitly specify which data type you want using the dtype keyword.

x = np.ones(2, dtype=np.int64)

To convert the type of an array, use the .astype()

z.astype(float)  # array([0.,  1.,  2.])

Fun Methods

Unittest

Method Checks that
assertEqual(a, b) a == b
assertNotEqual(a, b) a != b
assertTrue(x) bool(x) is True
assertFalse(x) bool(x) is False
assertIs(a, b) a is b
assertIsNot(a, b) a is not b
assertIsNone(x) x is None
assertIn(a, b) a in b
assertIsInstance(a, b) isinstance(a, b)
assertAlmostEqual(a, b) round(a-b, 7) == 0
assertGreater(a, b) a > b
assertGreaterEqual(a, b) a >= b
assertLess(a, b) a < b
assertLessEqual(a, b) a <= b
assertRegex(s, r) r.search(s)
assertMultiLineEqual(a, b) a == b, usable when a and b are strings
assertSequenceEqual(a, b) a == b, usable when a and b are lists, tuples, etc.
assertListEqual(a, b) a == b, usable when a and b are lists
assertTupleEqual(a, b) a == b, usable when a and b are tuples
assertSetEqual(a, b) a == b, usable when a and b are sets
assertDictEqual(a, b) a == b, usable when a and b are dicts
assertRaises(exc, fun, *args, **kwargs) fun(*args, **kwargs) raises exc

Functions that act on iterables

# `bool(item)` evaluates to `False` for each of these items
>>> any((0, None, [], 0))
False

# `bool(item)` evaluates to  `True` for each of these items
>>> all([1, (0, 1), True, "hi"])
True

Iterable Objects

Generators

Because range is a generator, the command range(5) will simply store the instructions needed to produce the sequence of numbers 0-4, whereas the list [0, 1, 2, 3, 4] stores all of these items in memory at once.

Python provides a sleek syntax for defining a simple generator in a single line of code; this expression is known as a generator comprehension.

# (<expression> for <var> in <iterable> if <condition>)
# when iterated over, `even_gen` will generate 0.. 2.. 4.. ... 98
even_gen = (i for i in range(100) if i%2 == 0)

Iterate

To create an object/class as an iterator you have to implement the methods __iter__() and __next__() to your object.

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    if self.a <= 20:
      x = self.a
      self.a += 1
      return x
    else:
      raise StopIteration

myclass = MyNumbers()
myiter = iter(myclass)

for x in myiter:
  print(x)

Method Overload

An efficient way to overload functions in Python

from multipledispatch import dispatch
 
# passing one parameter
@dispatch(int, int)
def product(first, second):
    result = first*second
    print(result)
 
# passing two parameters
@dispatch(int, int, int)
def product(first, second, third):
    result = first * second * third
    print(result)
 
# you can also pass data type of any value as per requirement
@dispatch(float, float, float)
def product(first, second, third):
    result = first * second * third
    print(result)
 
# calling product method with 2 arguments
product(2, 3)  # this will give output of 6
# calling product method with 3 arguments but all int
product(2, 3, 2)  # this will give output of 12
# calling product method with 3 arguments but all float
product(2.2, 3.4, 2.3)  # this will give output of 17.985999999999997

Flask

HTML Escaping

When returning HTML (the default response type in Flask), any user-provided values rendered in the output must be escaped to protect from injection attacks. HTML templates rendered with Jinja will do this automatically.

from markupsafe import escape

@app.route("/<name>")
def hello(name):
    return f"Hello, {escape(name)}!"

If a user managed to submit the name <script>alert("bad")</script>, escaping causes it to be rendered as text, rather than running the script in the user’s browser.

URL Building

To build a URL to a specific function, use the url_for() function. It accepts the name of the function as its first argument and any number of keyword arguments, each corresponding to a variable part of the URL rule. Unknown variable parts are appended to the URL as query parameters.

Use this in templates: ``

from flask import url_for

@app.route('/')
def index():
    return 'index'

@app.route('/login')
def login():
    return 'login'

@app.route('/user/<username>')
def profile(username):
    return f'{username}\'s profile'

with app.test_request_context():
    print(url_for('index'))
    print(url_for('login'))
    print(url_for('login', next='/'))
    print(url_for('profile', username='John Doe'))

Variable Rules

<converter:variable_name>

converters: string - int - float - path - uuid

Rendering Templates

from flask import render_template

@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
    return render_template('hello.html', name=name)

Flask will look for templates in the templates folder. So if your application is a module, this folder is next to that module, if it’s a package it’s actually inside your package.

Inside templates you also have access to the config, request, session and g objects as well as the url_for() and get_flashed_messages() functions.

Template Inheritance


{% extends "base.html" %}

{% block head %}
    <script defer type="module" src={{ url_for('static', filename='dist/main.js') }}></script>
    <link rel="stylesheet" href={{ url_for('static', filename='css/game.css') }}>
    <title>Connect-4</title>
{% endblock %}

{% block body %}
    <div id="app"></div>
{% endblock %}

HTTP Methods

By default, a route only answers to GET requests. You can use the methods argument of the route() decorator to handle different HTTP methods.

from flask import request

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        return do_the_login()
    else:
        return show_the_login_form()

@app.get('/login')
def login_get():
    return show_the_login_form()

@app.post('/login')
def login_post():
    return do_the_login()

Redirects

To redirect a user to another endpoint, use the redirect() function; to abort a request early with an error code, use the abort () function:

from flask import abort, redirect, url_for

@app.route('/')
def index():
    return redirect(url_for('login'))

@app.route('/login')
def login():
    abort(401)
    this_is_never_executed()

Error Handling

By default a black and white error page is shown for each error code. If you want to customize the error page, you can use the errorhandler() decorator:

from flask import render_template

@app.errorhandler(404)
def page_not_found(error):
    return render_template('page_not_found.html'), 404

Note the 404 after the render_template() call. This tells Flask that the status code of that page should be 404 which means not found. By default 200 is assumed which translates to: all went well.

Custom headers in response

You just need to wrap the return expression with make_response() and get the response object to modify it, then return it:

from flask import make_response

@app.errorhandler(404)
def not_found(error):
    resp = make_response(render_template('error.html'), 404)
    resp.headers['X-Something'] = 'A value'
    return resp

Request Object

from flask import request

The current request method is available by using the method attribute. To access form data (data transmitted in a POST or PUT request) you can use the form attribute. Here is a full example of the two attributes mentioned above:

@app.route('/login', methods=['POST', 'GET'])
def login():
    error = None
    if request.method == 'POST':
        if valid_login(request.form['username'],
                       request.form['password']):
            return log_the_user_in(request.form['username'])
        else:
            error = 'Invalid username/password'
    # the code below is executed if the request method
    # was GET or the credentials were invalid
    return render_template('login.html', error=error)

What happens if the key does not exist in the form attribute? In that case a special KeyError is raised.

If the data is JSON, the request should have the content-type header set to application/json and use request.json[]

To access parameters submitted in the URL (?key=value) you can use the args attribute:

searchword = request.args.get('key', '')

SSL

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

if __name__ == "__main__":
    app.run(ssl_context=('cert.pem', 'key.pem'))

$ flask run --cert=cert.pem --key=key.pem

Logging

Log Levels

Level When it’s used
DEBUG Detailed information, typically of interest only when diagnosing problems.
INFO Confirmation that things are working as expected.
WARNING An indication that something unexpected happened, or indicative of some problem in the near future (e.g. ‘disk space low’). The software is still working as expected.
ERROR Due to a more serious problem, the software has not been able to perform some function.
CRITICAL A serious error, indicating that the program itself may be unable to continue running.

Logging from multiple modules to file

# myapp.py
import logging
import mylib

def main():
    logging.basicConfig(filename='myapp.log', level=logging.INFO)
    logging.info('Started')
    mylib.do_something()
    logging.info('Finished')

if __name__ == '__main__':
    main()

# mylib.py
import logging

def do_something():
    logging.info('Doing something')

If you run myapp.py, you should see this in myapp.log:

INFO:root:Started
INFO:root:Doing something
INFO:root:Finished

logging to stderr

The simplest way to log to stdout using basicConfig:

import logging
import sys

logging.basicConfig(stream=sys.stdout, level=logging.DEBUG)

Python Pro Tips

  1. for-else 
     numbers = [2, 4, 6, 8, 1]
 for number in numbers:
     if number % 2 == 1:
         print(number)
         break
 else:
     print("No odd numbers")
  2. Get n largest or n smallest elements in a list using the module heapq 
     import heapq

 scores = [51, 33, 64, 87, 91, 75, 15, 49, 33, 82]
 print(heapq.nlargest(3, scores))  # [91, 87, 82]
 print(heapq.nsmallest(5, scores))  # [15, 33, 33, 49, 51]
  3. Pass values from a list as method arguments 
     my_list = [1, 2, 3, 4]
 print(my_list)  # [1, 2, 3, 4]
 print(*my_list)  # 1 2 3 4
  4. Get all the elements in the middle of the list 
     _, *elements_in_the_middle, _ = [1, 2, 3, 4, 5, 6, 7, 8]
 print(elements_in_the_middle)  # [2, 3, 4, 5, 6, 7]
  5. Enum 
     from enum import Enum

 class Status(Enum):
     NO_STATUS = -1
     NOT_STARTED = 0
     IN_PROGRESS = 1
     COMPLETED = 2

 print(Status.IN_PROGRESS.name)  # IN_PROGRESS
 print(Status.COMPLETED.value)  # 2
  6. Repeat strings without looping 
     string = "Abc"
 print(string * 5)  # AbcAbcAbcAbcAbc
  7. Merge dictionaries in a single readable line 
     first_dictionary = {'name': 'Fatos', 'location': 'Munich'}
 second_dictionary = {'name': 'Fatos', 'surname': 'Morina',
                     'location': 'Bavaria, Munich'}
 first_dictionary | second_dictionary  # {'name': 'Fatos', 'location': 'Bavaria, Munich', 'surname': 'Morina'}
 second_dictionary | first_dictionary  # {'name': 'Fatos', 'location': 'Munich', 'surname': 'Morina'}
  8. String to list 
     import ast

 def string_to_list(string):
     return ast.literal_eval(string)

 string = "[[1, 2, 3],[4, 5, 6]]"
 my_list = string_to_list(string)
 print(my_list)  # [[1, 2, 3], [4, 5, 6]]
  9. Print multiple values with a custom separator in between each value 
     print("29", "01", "2022", sep="/")  # 29/01/2022
  10. You can separate big numbers with the underscore 
    print(1_000_000_000)  # 1000000000
print(1_234_567)  # 1234567
  11. Find the unique id of a variable using id() 
    print(id(1))  # 4325776624
print(id(2))  # 4325776656
print(id("string"))  # 4327978288
  12. You can turn a set into an immutable set 
    my_set = frozenset(['a', 'b', 'c', 'd'])
my_set.add("a")  # AttributeError: 'frozenset' object has no attribute 'add'
  13. You can convert any data type into a boolean value 
    print(bool(.0))  # False
print(bool(3))  # True
print(bool("-"))  # True
print(bool("string"))  # True
print(bool(" "))  # True
  14. Convert a value into a complex number 
    print(complex(10, 2))  # (10+2j)
  15. You can also convert a number into a hexadecimal number: 
    print(hex(11))  # 0xb
  16. You don’t need to compare the length with 0 
    def get_element_with_comparison(my_list):
    if len(my_list) > 0:
        return my_list[0]

def get_first_element(my_list):
    if len(my_list):
        return my_list[0]
  17. Check the memory usage of an object 
    import sys
print(sys.getsizeof("bitcoin"))  # 56
  18. You can define a method that can be called with as many parameters as you want 
    def get_sum(*arguments):
    result = 0
    for i in arguments:
        result += i
    return result

print(get_sum(1, 2, 3))  # 6
print(get_sum(1, 2, 3, 4, 5))  # 15
print(get_sum(1, 2, 3, 4, 5, 6, 7))  # 28
  19. Magic methods for operators
    • __sub__() for -
    • __mul__() for *
    • __truediv__() for /
    • __ne__() for !=
    • __ge__() for >=
    • __gt__() for >
  20. Check if all characters in a string are either alphabets or numbers 
    name = "Password"
print(name.isalnum())  # True, because all characters are alphabets
name = "Secure Password "
print(name.isalnum())  # False, because it contains whitespaces
name = "S3cur3P4ssw0rd"
print(name.isalnum())  # True
  21. Remove characters from the right based on the argument 
    string = "This is a sentence with  "
# Remove trailing spaces from the right
print(string.rstrip())  # "This is a sentence with"
string = "this here is a sentence…..,,,,aaaaasd"
print(string.rstrip('.,dsa'))  # "this here is a sentence"
# lstrip for left
  22. Multiple conditions at a single if-statement 
    math_points = 51
biology_points = 78
physics_points = 56
history_points = 72

my_conditions = [math_points > 50, biology_points > 50,
                physics_points > 50, history_points > 50]

if all(my_conditions):
    print("Congratulations! You have passed all of the exams.")
else:
    print("I am sorry, but it seems that you have to repeat at least one exam.")
  23. Count the number of elements in a string or list using Counter from collections 
    from collections import Counter

result = Counter("Banana")
print(result)  # Counter({'a': 3, 'n': 2, 'B': 1})
result = Counter([1, 2, 1, 3, 1, 4, 1, 5, 1, 6])
print(result)  # Counter({1: 5, 2: 1, 3: 1, 4: 1, 5: 1, 6: 1})
  24. Find the most frequent element in a list in just one line 
    my_list = ['1', 1, 0, 'a', 'b', 2, 'a', 'c', 'a']
print(max(set(my_list), key=my_list.count))  # a

String Case Methods

  1. capitalize(): Capitalize the first character of a string.
  2. istitle(): Check if the first character of each word is capitalized.
  3. center()

Handwrite Notes

Vectorization

Vectorization is a technique to speed up your code by replacing explicit loops with array operations.

Read more examples in this source: Link

Using Loops

import time 
start = time.time()

# Iterating through DataFrame using iterrows
for idx, row in df.iterrows():
    # creating a new column 
    df.at[idx,'ratio'] = 100 * (row["d"] / row["c"])  
end = time.time()
print(end - start)
### 109 Seconds

Using Vectorization

start = time.time()
df["ratio"] = 100 * (df["d"] / df["c"])

end = time.time()
print(end - start)
### 0.12 seconds

Difference __repr__ and __str__

__repr__() provides the official string representation of an object, aimed at the programmer. __str__() provides the informal string representation of an object, aimed at the user.

os.path.exists(path) check if a file or directory exists.

Object to dict

The vars() built-in function can be used to convert an object into a dictionary in Python.

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

# Create an instance of Person
person = Person("Alice", 30)
# Convert the object to a dictionary using vars()
person_dict = vars(person)
# Print the resulting dictionary
print(person_dict)

Difference between sort() and sorted()

Difference between two dates

The d2 - d1 is timedelta object, which has a days attribute. Not other attributes like months or years!

from datetime import datetime

def days_between(d1, d2):
    d1 = datetime.strptime(d1, "%Y-%m-%d")
    d2 = datetime.strptime(d2, "%Y-%m-%d")
    return abs((d2 - d1).days)

Redis

import redis
r = redis.Redis(host='localhost', port=6379, decode_responses=True)

r.set('foo', 'bar')
# True
r.get('foo')
# bar

r.hset('user-session:123', mapping={
    'name': 'John',
    "surname": 'Smith',
    "company": 'Redis',
    "age": 29
})
# True
r.hgetall('user-session:123')
# {'surname': 'Smith', 'name': 'John', 'company': 'Redis', 'age': '29'}

All responses are returned as bytes in Python. To receive decoded strings, set decode_responses=True.

Environment Variable

# Retrieving the value of an environment variable with a default value
database_url = os.environ.get("DATABASE_URL", "localhost:5432")
# Setting a new environment variable
os.environ["API_KEY"] = "YOUR_API_KEY"

Working With Dates

Add, Subtract:

from datetime import datetime, timedelta
print("Current time: ", datetime.now())
print(datetime.now() - timedelta(days=5, hours=-5))

strftime() and strptime():

from datetime import datetime
now = datetime.now()
print(now.strftime("%Y-%m-%d %H:%M:%S")) # convert datetime to string
print(datetime.strptime("2021-01-01 12:00:00", "%Y-%m-%d %H:%M:%S")) # convert string to datetime

Reflection