Skip to content

Instantly share code, notes, and snippets.

@hay

hay/cheatsheet.md

Last active January 6, 2021 15:19
Show Gist options
  • Save hay/a09e180c6064906b5271 to your computer and use it in GitHub Desktop.
Save hay/a09e180c6064906b5271 to your computer and use it in GitHub Desktop.
Download ZIP
Hay's dev cheatsheet
Raw
cheatsheet.md

Hay's dev cheatsheet

So, how do i...

Index

Web development

Fixing Internet Explorer 11 bugs

Add a class if the browser is Internet Explorer 11 or lower

if ('ActiveXObject' in window) {
    document.documentElement.className = 'is-ie';
}

Fixing childen of a flexbox container exceeding their parent width

Just add width: 100% to the elements, or if you're really lazy:

.container > * {
    width: 100%;
}

SCSS

Getting both value and index from a SASS list

$colors: red, blue, yellow, green;
@for $index from 1 through length($colors) {
    li:nth-child(#{length($colors)}n + #{$index}):before {
        background-image: url('../img/bullet-#{nth($colors, $index)}.svg');
    }
}

Javascript

Is there a quicker way to assign a default value if the result is undefined?

Instead of this:

const bar = foo.bar ? foo.bar : 'bar';

Or this:

let bar;

if (foo.bar) {
  bar = foo.bar;
} else {
  bar = 'bar';
}

Try this

const bar = foo.bar || 'bar';

Promises

// Transforming jQuery's $.get to a Promise
function get(url) {
    return new Promise(function(resolve, reject) {
        $.ajax({
            method : "GET",
            url : url,
            success : function(data) {
                resolve(data);
            },
            error : function() {
                reject('Something went wrong');
            }
        }
    });
}

get('http://example.com').then(function(data) {
    console.log(data);
});

async/await

const URL = 'https://api.github.com/gists/a09e180c6064906b5271';

async function getGist() {
    try {
        const res = await fetch(URL);
        const data = await res.json();
        return data;
    } catch (err) {
        console.log(err);
    }
}

getGist().then(gist => console.log(gist.description));

Lodash

Transform an array with objects to an object with keys and values

_.mapValues(_.keyBy(data, 'key'), 'value');

Transform this:

[
    { key : 'foo', value : 'bar'},
    { key : 'baz', value : 'qux'}
]

Into this:

{
    'foo' : 'bar',
    'baz' : 'qux'
}

PHP

Get the path of the current script

echo __FILE__;

Get the directory of the current script

echo realpath(dirname(__FILE__));

Fix those pesky PDO SQLSTATE[HY000] [2002] No such file or directory errors

  • Try using 127.0.0.1 instead of localhost for DB host
  • Add a utf-8 charset

Do a 301 permanent redirect

header("Location: https://www.example.com", true, 301);

Git

Check in an empty directory

Create a .gitignore with this content:

# Ignore everything in this directory
*
# Except this file
!.gitignore

Add something to your last commit

Use git add just as you would for a normal commit. Then:

git commit --ammend

Edit last commit message

git commit --amend -m "New commit message"

Resolve all merge conflicts by overwriting with all local or remote files (source)

To use local files

grep -lr '<<<<<<<' . | xargs git checkout --ours

To use remote files

grep -lr '<<<<<<<' . | xargs git checkout --theirs

Remove local (untracked) files from a current branch

git clean -f

Directories as well

git clean -fd

.gitignore files as well

git clean -fdx

Show a single file from a specific revision

git show <revision>:<filename>

Remove an already pushed tag in a Github / remote repo

git tag -d yourtag
git push origin :refs/tags/yourtag

Undo the last commit

git reset --hard HEAD^

If you don't care about the changes, or

git reset --soft HEAD^

If you do.

If you've already pushed, try this:

git revert HEAD

Show all files that will get added when doing a git add .

git status -u

Python

Dicts and lists

Iterate over keys and values in a dict

obj = { "key" : val }

# Python 2
for key, val in obj.iteritems():
    print key, val

# Python 3
for key, val in obj.items():
    print(key, val)

Extend / merge a dict with more values

a = { "foo" : 1 }
a.update({ "bar" : 2 })

print(a) # { "foo" : 1, "bar" : 2 }

Get the first item in a dict

a = { "a" : {}, "b" : {} }
a.itervalues().next()

Get the first N items in a dict

{k:v for k,v in list(d.items())[0:N]}

Remove a key from a dict

my_dict.pop("key", None)

Get a default value for a dict if the key does not exist

a = { "foo" : "foo" }
a["foo"] // "foo"
a.get("bar", "bar") // "bar"
a["bar"] // KeyError: "bar"

Get the index number when looping over a list (array)

l = [1,2,3]
for index, val in enumerate(l):
    print(index, val) # "0, 1", "1, 2", "2, 3"

Sort a list by a user defined function

a = [ { "foo" : "bar" }, {"foo" : "baz"} ]

a.sort(key = lambda i:i["foo"])

Or

a = sorted(a, key = lambda i:i["foo"])

Sort a list with dicts by a key in the dicts?

a = [ { "name" : "Bert" }, { "name" : "Ernie" }

from operator import itemgetter
b = sorted(a, key = itemgetter("name") )

Sort a list with lists on one of the items?

a = [ ["foo", 3], ["foo", 1], ["foo", 2] ]
a.sort(key = lambda i:i[1]) // [ ["foo", 1], ["foo", 2], ["foo", 3] ]

Sort a dict by its values?

x = { "a" : 3, "b" : 2, "c" : 9 }
dict(sorted(x.items(), key=lambda item: item[1]))

Get the key of a dict where the value is the largest

a = { "foo" : 10, "bar" : 20, "baz" : 5 }
max(a, key = (lambda k:a[k])) # 'bar'

Count the number of times an item appears in a list?

from collections import Counter

l = ["foo", "bar", "foo", "baz"]
print(Counter(l))

# Prints:
# {'foo': 2, 'baz': 1, 'bar': 1}

Flatten a list with nested lists?

The ugly way

sum(l, [])

The slighty more Pythonic (and faster) way

[item for sublist in l for item in sublist]

And the fastest way

from itertools import chain
list(chain.from_iterable(l))

Get the unique values in a list

Convert to a set, then to a list again

list(set(l))

Remove falsy values from a list

Python 2

values = filter(None, [1, None, 3])

Python 3

values = list(filter(None, [1, None, 3]))

Split a list in evenly sized chunks

lst = list(range(0, 180))

chunks = [ lst[i:i + 50] for i in range(0, len(lst), 50) ]

Or if you want a function:

def batch(iterable, n = 1):
    for i in range(0, len(iterable), n):
        yield iterable[i:i + n]

Filter a list using comprehensions

l = [5, 3, 7, 24, 2, 22]

l = [ x for x in l if x > 10 ] # l == [24, 22]

Dict comprehensions

d = {k:v for k,v in dct.iteritems() } # Python 2
d = {k:v for k,v in dct.items() } # Python 3

Strings

Format a string

Either

"Ham, %s and %s" % ("eggs", "spam")

Or

"Ham, {food1} and {food2}".format(food1 = "eggs", food2 = "spam")

Or (since Python 3.6)

food1 = "eggs"
food2 = "spam"
f"Ham, {food1} and {food2}"

Pad a number with zeroes

If the number is already a string

number.zfill(3)

URL encode a string

Python 2 urllib.quote_plus("Lots of weird charachters:ª•¶•ª¶™•ª¢∆")

Python 3 urllib.parse.quote_plus("Lots of weird charachters:ª•¶•ª¶™•ª¢∆")

Check if any string in a list appears in another string

target = "a lovely string"
strings = ("nice", "kind", "lovely")

if any(s in target for s in strings):
    print(s) # 'lovely'

Regular expressions

import re

REGEX = re.compile("…[^…]*…")

# Replace all occurences
text = REGEX.sub(" ", text)

# Get all matches
matches = REGEX.findall(text)

# Split by regex
parts = REGEX.split(text)

Dates and times

Get the current time as a ISO8601-formatted string

import datetime
datetime.datetime.now().isoformat("T")

Files

Get the directory of the current executing file

import os
PATH = os.path.dirname(os.path.realpath(__file__))

In reverse

a.sort(key = lambda i:i[1], reverse = True) // [ ["foo", 3], ["foo", 2], ["foo", 1] ]

Check if a file exists?

import os.path
os.path.isfile( filename )

Get a filename without extension

import os
os.path.basename(filename)[0]

To also remove the path to the filename

os.path.splitext(os.path.basename(filename))[0]

Logging

import logging
logger = logging.getLogger(__name__)

logger.critical("Log with critical level")
logger.error("Log with error level")
logger.warning("Log with warning level")
logger.info("Log with info level")
logger.debug("Log with debug level")

logging.basicConfig(level=logging.DEBUG) # Set default level to DEBUG

Classes

class Parent:

    foo = "Unknown"

    def __init__(self, foo):
        self.foo = foo

    def bar(self):
        print(self.foo)

class Child(Parent):
    def __init__(self, foo):
        super().__init__(foo)

Use __getitem__ for easy list access

class Uppercaser:
    def __getitem__(self, key):
        return key.upper()

up = Uppercaser()
print(up["hello"]) # HELLO

Use properties (getters and setters)

class Namer:
    def __init__(self, name):
        self._name = name

    @property
    def name(self):
        return self._name.upper()

    @name.setter
    def name(self, value):
        self._name = value

Errors and Exceptions

def test():
    raise Exception("Something happened")

try:
    test()
except Exception as e:
    print(e.message)
else:
    # Do something else

Note that you can re-raise an exception

try:
    test()
except Exception as e:
    raise e

Custom exceptions

    class MyException(Exception):
        pass

    try:
        raise MyException("oops")
    except MyException as e:
        print("oops")

XML

To parse XML use xmltodict, to make it behave like a dict.

import xmltodict

doc = xmltodict.parse("""
<record id="123">
    <key>foo</key>
</record>
""")

print(doc["record"]["@id"]) # '123'
print(doc["record"]["key"]) # foo

To write back XML you can use xmltodict's unparse

print(xmltodict.unparse(doc, pretty = True)) # <record id="123">....

Or use lxml.etree.tostring()

from lxml import etree
print(etree.tostring(doc))

argparse

import argparse

parser = argparse.ArgumentParser(description = "Pretty description here")
parser.add_argument('head', type = str, nargs='?')
parser.add_argument('face', type = str, nargs='?')
parser.add_argument('-o', '--output', type = str, default = "out.jpg")
parser.add_argument('-v', '--verbose', action="store_true")
parser.add_argument('-c', '--caching', choices=('default', 'redis'))
args = parser.parse_args()

if not args.head and args.face:
    ArgumentParser.print_help()

Other stuff

Simplify a complex loop?

Use a generator. Instead of

for point in points:
    for x in point:
        if x > 5:
            plot(x)

Do

def iterpoints():
    for point in points:
        for x in points:
            if x > 5:
                yield x

for x in iterpoints():
    plot(x)

Increment an integer? foo++ doesn't work. (source)

foo += 1

Or

foo = foo + 1

Break into debugger from the code itself

import pdb;pdb.set_trace()

Encode URLs?

from urllib import quote_plus as encode
encode('Something with spaces and ••ª¶ª•¶∆˙˚∆ charachters')

Transform a CSV file using dicts

import csv

# Get the fieldnames first
infile = open("in.csv", "r")
reader = csv.DictReader(infile)
fieldnames = reader.next().keys()
infile.close()

# Now open the in and outfile
infile = open("in.csv", "r")
outfile = open("out.csv", "w")
reader = csv.DictReader(infile)
writer = csv.DictWriter(outfile, fieldnames = fieldnames)

# Print the field names in the first row
writer.writerow(
    dict(
        (fn, fn) for fn in fieldnames
    )
)

for row in reader:
    # Do something with the values

    # And then write it
    writer.writerow(row)

infile.close()
outfile.close()

How do i remove the mysterious "ValueError: View function did not return a response" with WSGI?

Check if there's no print statements in your code.

Parsing HTML using BeautifulSoup

from bs4 import BeautifulSoup

soup = BeautifulSoup(html, 'html5lib')
videos = soup.select(".video")

for v in videos:
    stats = v.select_one(".stats").get_text()

    return {
        "stats" : stats,
        "url" : v.get("href", None),
        "title" : v.select("h1")[0].get_text(),
    }

Download a file over HTTP

import urllib.request
urllib.request.urlretrieve('http://www.example.com/songs/mp3.mp3', 'mp3.mp3')

Execute a shell command

import subprocess
subprocess.check_call(cmd, shell = True)

To do a 'silent' call, without any logging

import os, subprocess
subprocess.check_call(cmd, shell = True,
    stdout = open(os.devnull, 'wb')
)

Virtualenv

Make sure virtualenv is installed

pip install virtualenv

This will create an venv in a directory called env

virtualenv env

Then to activate it

source env/bin/activate

Installing local libs (from the venv)

pip install /path/to/lib

Or use venv in python3

python3 -m venv env
source env/bin/activate
pip install -r requirements.txt

Multithreading

from concurrent.futures import ThreadPoolExecutor

def square(val):
    return val * val

values = [5, 4, 12, 40]

with ThreadPoolExecutor(max_workers = 10) as executor:
    executor.map(square, values)

Command line (for *nix-likes)

  • See this excellent resource.

Login with SSH to an unreachable server via another server

Host          internal.hostname.tld
User          user
HostName      internal.hostname.tld
ProxyCommand  ssh [email protected] nc %h %p 2> /dev/null

Weird rsync errors

Errors like this could be anything:

rsync error: error in rsync protocol data stream

But make sure your SSH keys have proper permissions

chmod -R 700 ~/.ssh

Convert a batch of image files

Use mogrifyto convert a bunch of JPG files to fit within a 1500x1500 frame (take longest side), and set quality to 60%

mogrify -resize 1500x1500 -quality 60 *.jpg

Same thing, but crop the images as well so they fit in the 1500x1500 frame, cropped from the center of the image.

mogrify -resize 1500x1500^ -crop 1500x1500+0+0 -gravity center *.jpg

Convert pages from a multi-page PDF to separate JPG files (3000px longest side), trimming excess whitespace

convert -resize 3000x doc.pdf -trim image-%d.jpg

Convert a set of image files to a PDF

convert *.png doc.pdf

Yes, it really is that simple.

Sequentially rename a bunch of files

Use the rename command to sequentially number a bunch of JPG files to 01.jpg, 02.jpg, etc.

rename -N 01 's/.*/$N.jpg/' *

Append both stdout and stderr to a file in Bash

command >> logfile.log 2>&1

Video converting

Basic use of ffmpeg

ffmpeg -i source.extension dest.extension

Resize to 600px wide

ffmpeg -i in.mov -vf scale=600:-1 out.mp4

Crop a video with landscape aspect-ratio to a square from the center

ffmpeg -i in.mov -filter:v "crop=in_h:in_h:((in_w/2)-(in_h/2)):0" out.mp4

Framerate to 24fps

ffmpeg -i in.mov -r 24 out.mp4

Bitrate to 2mb/s

ffmpeg -i in.mov -b:v 2000k out.mp4

Grab a frame from 1 second into the video and convert to jpg (note that .png also works)

ffmpeg -i in.mov -ss 00:00:01 -vframes 1 out.jpg

Make sure it works in Quicktime

ffmpeg -i in.mov -pix_fmt yuv420p out.mp4

Convert from a movie to a series of JPG's (note that -q:v 2 is only needed for JPG)

ffmpeg -i clip.mp4 -q:v 2 "frames/%04d.jpg"

Grab a frame from every second of a clip and save to jpg

ffmpeg -i clip.mp4 -vf fps=1 frames/frame-%d.jpg

Same, but now every minute

ffmpeg -i clip.mp4 -vf fps=1/60 frames/frame-%d.jpg

Convert a bunch of images to a movie clip

ffmpeg -r 24.89 -f image2 -s 1280x720 -i "frames/%04d.jpg" -vcodec libx264 -crf 25 -pix_fmt yuv420p movie.mp4

Convert a MP3 to a MP4 with a single image

ffmpeg -loop 1 -y -i image.jpg -i music.mp3 -shortest -pix_fmt yuv420p output.mp4

Add MP3 audio to a movie and save as MP4 (given the audio is wav, i had less luck with mp3)

ffmpeg -i a.mp4 -i a.wav -c copy -map 0:v:0 -map 1:a:0 -shortest -c:a aac -b:a 192k b.mp4

Fix out of sync audio/video (note itsoffset parameter)

ffmpeg -i original.avi -itsoffset 0.2 -i original.avi -map 0:0 -map 1:1 -acodec copy -vcodec copy synced.avi

Download movies from YouTube and other video services

Use the brilliant youtube-dl.

youtube-dl -f mp4 https://www.youtube.com/watch?v=bMj3Pc5I078

Directly convert a movie at an URL to mp3

youtube-dl -x --audio-format mp3 https://www.youtube.com/watch?v=bMj3Pc5I078

Batch run a command on all files in a directory

for f in *.ext; do command $f; done

Search for a string appearing in files in directory and all subdirectoris, ignoring case

grep -lir some_string .

The same, but only in files with a certain extension

grep -lir --include \*.py some_string .

Get total size of directories, reporting only on the current level

du -h -d1

Query DNS records

Query the A record for example.com

dig example.com A

Query the MX record(s) for example.com

dig example.com MX

Get the result as just the IP instead of the whole query/answer stuff

dig example.com A +short

Comand line (Linux)

Fix those stupid bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8) warnings

$ vim /etc/locale.gen

Uncomment the en_US.UTF-8 line

$ locale-gen

If you're still getting errors, add these two lines to your bashrc or bash_profile on the machine you're connecting from:

# Setting for the new UTF-8 terminal support in Lion
export LC_CTYPE=en_US.UTF-8
export LC_ALL=en_US.UTF-8

Add user to group

usermod -aG group user

Command line (Mac OS X specific)

Pipe outputs to the clipboard

ls | pbcopy

Find stuff on the command line using spotlight

mdfind

Chrome

Remove URLS from autocomplete

Highlight the item using keyboard arrows and press Shift and Delete keys.

Spreadsheets

Use a formula on the output of a condition

=AVERAGE(FILTER(A2:A25, A2:A25>0))

Select a complete row or column in a formula

=AVERAGE(B:B)

Count the number of cells

=COUNTA(B:B)

Only count cells that meet a certain condition

=COUNTIF(B:B, "Yes")

Get the data from a cell with more than one condition (use shift-ctrl-enter to activate formula, note the ampersands)

=INDEX(C:C,MATCH(B2&"value",A:A&B:B,0))

(Excel only): convert seconds to a hh:mm:ss notation

Divide the number of seconds by 86400 (number of seconds in a day) --> format cell --> custom --> enter 'hh:mm:ss'

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment