Python Cheat Sheet

Python Cheat Sheet

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1. Installation/Config

1. 1. PIP

```shell -- list all installed packages pip list

-- list only local installed packages in a virtual env pip list --local

-- search a package pip list|grep <packagename>

-- show the package location pip show <packagename>

-- location of the globally installed packages python -m site

>>> import site >>> print(site.getsitepackages())'

or >>> import sys >>> sys.path

-- location of the locally installed packages pythom -m site --user-site -- ```

1. Virtual Env (Python 3.3+)

venv is a package shipped with Python 3, which you can run using python3 -m venv. It serves the same purpose as virtualenv, but only has a subset of its features. virtualenv continues to be more popular than venv, especially since the former supports both Python 2 and 3.

[Difference between virtualenv and venv](https://virtualenv.pypa.io/en/latest/)

1. 1. Install virtualenv

```shell pip install virtualenv

-- upgrade virtualenv pip install --upgrade virtualenv

-- using virtualenv to create a virtual env for a particular python version virtualenv -p /usr/local/bin/python3.5 kevin-venv ```

1. 1. Create a virtual env

```Shell python -m venv <venv-name>

python -m venv kevin-venv

-- include global python packages python -m venv kevin-venv --system-site-packages

-- using --local to list only local installed packages pip list --local

-- typically, you create a folder for your project first; -- then create a virtual env within/insider of that project folder >1. mkdir kevin_python_proj1 -- 2a and 2b is same; pick one you like >2a. python -m venv kevin_python_proj1/.venv >2b. cd mkdir kevin_python_proj1 && python -m venv .venv

```

1. 1. Activate/Deactivate the vtirual env

```Shell -- activate source <venv-name>/bin/activate source kevin-venv/bin/activate

-- deactivate deactivate ```

1. 1. Delete a virtualenv

```shell rm -rf <venv-name> rm -rf kevin-venv ```

1. 1. Store copy of python package versions

```shell -- all packages pip freeze > requirements.txt -- only local installed packages pip freeze --local > requirements.txt ```

1. 1. Install all packages from requirements.txt

```shell pip install -r requriments.txt ```

1. Tips and Tricks

1. 1. Date, Time, DateTime, Timezone

[See this](https://docs.python.org/3/library/datetime.html#datetime.datetime.now)

```python from datetime import datetime, date, time, timezone, timedelta

current_time = datetime.now(timezone.utc) print("The current time with UTC is: ", current_time.strftime('%Y-%m-%d %H:%M:%S %Z'))

The current time with UTC is: 2022-02-15 21:50:52 UT ==> when I run this from laptop, it is 3:50:52 PM,
this mean Chicago is UTC-6

yesterday = date.today() - timedelta(days=1) dt = yesterday.strftime("%Y-%m-%d") print("Yesterday is: ", dt)

Yesterday is: 2022-02-14

local time

print("The local time is: ", time.strftime("%Y-%m-%d %H:%M:%S", time.localtime()));

The local time is: 2022-02-15 15:50:03

```

1. 1. switch value

```python a = 10 b = 30

switch value, now a = 30, b = 10

a,b = b,a ```

1. 1. Merging Dictionaries (Python 3.5+)

```python x = {'a': 1, 'b': 2} y = {'c', 3, 'd': 4}

merged = {'a': 1, 'b': 2, 'c': 3, 'd': 4}

merged = {**x, **y}

if have same key, then right one win

x = {'a': 1, 'b': 2} y = {'a', 3, 'c': 4}

merged = {'a': 3, 'b': 2, 'c': 4}

merged = {**x, **y} ```

1. 1. Check if a module contain a class/method

```python

Using a list comprehension

import os [name for name in dir(os) if 'uname' in name.lower()] # return ['uname', 'uname_result']

After you find the class, you can use built-in help() to see the detail

help(os.uname)

another example

import collections [name for name in dir(collections) if 'tuple' in name.lower()] # return ['_tuplegetter', 'namedtuple']

help(collections.namedtuple) ```

1. Comprehensions

1. 1. List Comprehensions

[expression for item in iterable] [expression for item in iterable if condition]

```python squares = [x * x for x in range(10)]

even_squares = [x * x for x in range(10) if x % 2 == 0]

multiple loops

x_dimension = ['a', 'b'] y_dimension = [1, 2, 3] matrix = [(x,y) for x in x_dimension for y in y_dimension]

output: [('a', 1), ('a', 2), ('a', 3), ('b', 1), ('b', 2), ('b', 3)]

```

1. 1. Tuple Comprehensions

there is no such thing. there is no tuple comprehension

1. 1. Dictionary Comprehensions

{key_expression: value_expression for expression in iterable}

```python word = "dictionary"

Method A:

letter_count1 = {letter: word.count(letter) for letter in word}

Method B: better performance using set() to remove duplicated letter from word first

letter_count2 = {letter: word.count(letter) for letter in set(word)}

with condition

old_salary = {'Kevin': 2000, 'Brandon': 400, 'Elena': 300} new_salary = {k: v*1.5 if v < 500 else v for (k, v) in old_salary.items()}

output: new_salary = {'Kevin': 2000, 'Brandon': 600.0, 'Elena': 450.0}

```

1. 1. Set Comprehensions

{expression for item in iterable} {expression for item in iterable if condition}

```python random_nums = [3, 6, 9, 3, 9, 18, 20, 18]

great_tens = {num for num in random_nums if num > 10}

output: {18, 20}

```

1. 1. Generator Comprehensions

```python from pprint import pprint some_nums = (num for num in range(5)) pprint(some_nums)

output: <generator object <genexpr> at 0x00000264B87BCF90>

```

1. Functional Programming

1. 1. Named Tuple

```python import collections from pprint import pprint

Cruise = collections.namedtuple(

   "Cruise",
   [
       "year",
       "cruise_line",
       "ship",
       "days",
   ],

)

my_cruise = (

   Cruise(year=2016, cruise_line="RCL", ship="Independency of Seas", days=5),
   Cruise(year=2017, cruise_line="NCL", ship="Escape", days=7),
   Cruise(year=2018, cruise_line="NCL", ship="Bliss", days=7),

)

pprint(my_cruise)

output

((Cruise(year=2016, cruise_line='RCL', ship='Independency of Seas', days=5),

Cruise(year=2017, cruise_line='NCL', ship='Escape', days=7),
Cruise(year=2018, cruise_line='NCL', ship='Bliss', days=7))

```

1. 1. Filter()

```python

Method A: print only RCL cruise: using filter

rcl_cruise = tuple(

   filter(lambda cruise: cruise.cruise_line.upper() == "RCL", my_cruise)

)

pprint(rcl_cruise)

output

(Cruise(year=2016, cruise_line='RCL', ship='Independency of Seas', days=5),)

Method B: print only RCL cruise: using iterator

rcl_cruise2 = tuple(

   cruise for cruise in my_cruise if cruise.cruise_line.upper() == "RCL"

)

pprint(rcl_cruise2) ```

1. 1. Map Function

```python

1. Method A: using map function

cruise_history = tuple(

   map(
       lambda cruise: {
           "cruise_line": cruise.cruise_line,
           "name": cruise.ship,
           "past_year": 2022 - cruise.year,
       },
       my_cruise,
   )

)

pprint(cruise_history)

({'cruise_line': 'RCL', 'name': 'Independency of Seas', 'past_year': 6},
{'cruise_line': 'NCL', 'name': 'Escape', 'past_year': 5},
{'cruise_line': 'NCL', 'name': 'Bliss', 'past_year': 4})

Method B: using generator comprehension and convert it to tuple

cruise_history2 = tuple(

   {
       "cruise_line": cruise.cruise_line,
       "name": cruise.ship,
       "past_year": 2022 - cruise.year,
   }
   for cruise in my_cruise

)

Another great example: track for each cruise line, which year I took it

Method A: hard-coded the cruise line dictionary object as initial value for reduce() function

def reducer(acc, val):

   acc[val.cruise_line].append(val.year)
   return acc

cruise_line_year_history = reduce(reducer, my_cruise, {"RCL": [], "NCL": [], "DISNEY": []})

pprint(cruise_line_year_history)

{'DISNEY': [], 'NCL': [2017, 2018], 'RCL': [2016]}

Method B: improve on top of Method A using collections.defaultdict

import collections

cruise_line_year_history2 = reduce(reducer, my_cruise, collections.defaultdict(list))

pprint(cruise_line_year_history2)

defaultdict(<class 'list'>, {'RCL': [2016], 'NCL': [2017, 2018]})
please notice result doesn't have 'DISNEY' in it

```

1. 1. Reduce Function

```python

Method A: using reduce() function

from functools import reduce total_cruise_days = reduce(lambda acc, cruise: acc + cruise.days, my_cruise, 0)

pprint(total_cruise_days)

output: 19

Method B: using sum() function instead of reduce()

total_cruise_days2 = sum(cruise.days for cruise in my_cruise) ```

1. 1. Parallel Processing

Please watch below youtube by Dan Bader

- [multiprocessing](https://www.youtube.com/watch?v=aysceqdGFw8&list=PLP8GkvaIxJP1z5bu4NX_bFrEInBkAgTMr)

- [concurrent.futures](https://www.youtube.com/watch?v=0NNV8FDuck8&list=PLP8GkvaIxJP1z5bu4NX_bFrEInBkAgTMr)

```python

Method A

import multiprocessing

Method B

import concurrent.futures ```

1. Class & Object

1. 1. Dynamic property set and get: setattr()/getattr()

```python class Cruise():

   pass

cruise_2019 = Cruise()

first_key = 'cruise_line' first_val = 'RCL Symphony'

second_key = 'cruise_days' second_val = 7

setattr(cruise_2019, first_key, first_val) setattr(cruise_2019, second_key, second_val)

print(cruise_2019.cruise_line, cruise_2019.cruise_days)

RCL Symphony 7

cruise_2019_days = getattr(cruise_2019, second_key) print(cruise_2019_days)

convert dictionary to class object

cruise_2018_dict = {'cruise_line': 'NCL Bliss', 'cruise_days': 7} cruise_2018 = Cruise()

for k, v in cruise_2018_dict.items():

   setattr(cruise_2018, k, v)

for k in cruise_2018_dict.keys():

   print(getattr(cruise_2018, k))

NCL Bliss
7

```

1. List

1. 1. Looping List with both index and item: enumerate

```python cruise_hist = ['NCL Bliss', 'NCL Escape', 'Disney Dreamer']

for idx, cruise in enumerate(cruise_hist):

   print(f"{idx}: {cruise}")

Test

0: NCL Bliss 1: NCL Escape 2: Disney Dreamer

using start=1

cruise_hist = ['NCL Bliss', 'NCL Escape', 'Disney Dreamer']

for idx, cruise in enumerate(cruise_hist, start=1):

   print(f"{idx}: {cruise}")

Test

1: NCL Bliss 2: NCL Escape 3: Disney Dreamer ```

1. 1. Loop multiple lists using zip()

```python cruise_lines = ['NCL Bliss', 'RCL Independency', 'Disney Dreamer'] cruise_days = [7, 5, 4] cruise_years = [2018, 2016, 2015]

for cruise_line, cruise_day, cruise_year in zip(cruise_lines, cruise_days, cruise_years):

   print(
       f"I cruised with {cruise_line} in year {cruise_year} for {cruise_day} days")

Test

I cruised with NCL Bliss in year 2018 for 7 days I cruised with RCL Independency in year 2016 for 5 days I cruised with Disney Dreamer in year 2015 for 4 days

cruise_tuple is a tuple in below

for cruise_tuple in zip(cruise_lines, cruise_days, cruise_years):

   print(f"I cruised with {cruise_tuple} ")

Test

I cruised with ('NCL Bliss', 7, 2018) I cruised with ('RCL Independency', 5, 2016) I cruised with ('Disney Dreamer', 4, 2015)

```

1. File Operation

1. 1. Using Content Manager

```python with open('test.txt', 'rd') as f:

   file_contents = f.read()

words = file_contents.split(' ') word_count = len(words) ```

1. Misc

1. 1. Get Help

- In Python Interpreter, type 'help(dir)' - In Python Interpreter, type build-in 'dir(xyz)'

Make sure you import the module(xyz) you want to investigate before you run help(xyz) or dir(xyz)

1. 1. Input secret information (e.g. password)

```python from getpass import getpass

username = input('Username: ') password = getpass('Password: ') ```

1. 1. Unpacking

```python cruise_2018 = ('NCL Bliss', 7)

using _ as a variable is a convention in Python
to indicate you are not use that variable

_, days_cruised_2018 = cruise_2018

print(days_cruised_2018) # 7 ```

```python cruise_years = [2008, 2015, 2016, 2017, 2018, 2019]

first_cruise_yr, second_cruise_yr, *rest_cruise_yrs = cruise_years

print(f"my very 1st cruise is in year {first_cruise_yr}") print(f"my 2nd cruise is in year {second_cruise_yr}") print(f"my rest of cruise years are {rest_cruise_yrs}")

Test

my very 1st cruise is in year 2008 my 2nd cruise is in year 2015 my rest of cruise years are [2016, 2017, 2018, 2019] ```

```python cruise_years = [2008, 2015, 2016, 2017, 2018, 2019]

first_cruise_yr, *rest_cruise_yrs, latest_cruise_yr = cruise_years

print(f"my very 1st cruise is in year {first_cruise_yr}") print(f"my latest cruise year is ==> {latest_cruise_yr}") print(f"my rest of cruise years are {rest_cruise_yrs}")

Test

my very 1st cruise is in year 2008 my latest cruise year is ==> 2019 my rest of cruise years are [2015, 2016, 2017, 2018] ```

1. 1. Ternary Operator

```python

cruise_2018 = 'NCL' cruise_2019 = 'RCL'

favorite_flag = 'Y' if cruise_2019 == my_favorite_cruise else 'N' print(f"my 2019 cruise is my favorite cruise? {favorite_flag}")

favorite_flag = 'Y' if cruise_2018 == my_favorite_cruise else 'N' print(f"my 2018 cruise is my favorite cruise? {favorite_flag}")

Test

my 2019 cruise is my favorite cruise? Y my 2018 cruise is my favorite cruise? N ```

1. 1. Large Number format

- Using underscore (\_) in declaration - Using ':," to set group seperator to ',' in f string

```python china_population = 1_300_000_000_000 usa_population = 330_000_000_000

print(f"China population = {china_population:,}") print(f"USA {usa_population:,}")

Test

China population = 1,300,000,000,000 USA 330,000,000,000 ```

Python Cheat Sheet

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Python Cheat Sheet

DB CAFE

Dbcafe (토론 | 기여)님의 2023년 11월 27일 (월) 23:29 판(차이) ← 이전 판 | 최신판 (차이) | 다음 판 → (차이)

Dbcafe (토론 | 기여)님의 2023년 11월 27일 (월) 23:29 판
(차이) ← 이전 판 | 최신판 (차이) | 다음 판 → (차이)