snippets.md

Useful Python code snippets

A collection of several code snippets of common or useful operation in Python.

Index

• Get list of attributes (methods) of an object
• Remove dictionary from list
• List Comprehension
• Lambda Functions
• Map and Filter
• Arange and Linspace
• Download image from url
• Create centered thumbnail from image
• Handle keyboard interrupt (Ctrl + C)
• Merge dictionaries

Get list of attributes (methods) of an object

Get a list of attributes (methods) of an object with dir(object)

To see everything except hidden/dunder methods you can do:

print((d for d in dir(obj) if not d.startswith('_')))

Remove dictionary from list

Here's how to remove a specific dictionary, or several dictionaries, from a list of dictionaries by a specific key. In the example below we are removing from the list all dictionaries which have and id key of 2.

thelist = [{'id': 1, 'name': 'paul'},
           {'id': 2, 'name': 'john'}]
           
thelist[:] = [d for d in thelist if d.get('id') != 2]

List Comprehension

Without list comprehension:

x = [1,2,3,4]
out = []
for item in x:
    out.append(item**2)
print(out)
# [1, 4, 9, 16]

With list comprehension:

x = [1,2,3,4]
out = [item**2 for item in x]
print(out)
# [1, 4, 9, 16]

Lambda Functions

Lambda functions are used for creating small, one-time and anonymous function objects in Python. Basically, they let you create a function, without creating a function. Note that lambda functions can do everything that regular functions can do, as long as there’s just one expression. The basic syntax of lambda functions is: lambda arguments: expression. And here is an example:

double = lambda x: x * 2
print(double(5))
# 10

Map and Filter

Once you have a grasp on lambda functions, learning to pair them with the map and filter functions can be a powerful tool. Specifically, map takes in a list and transforms it into a new list by performing some sort of operation on each element. In this example, it goes through each element and maps the result of itself times 2 to a new list. Note that the list function simply converts the output to list type.

seq = [1, 2, 3, 4, 5]
list(map(lambda var: var*2, seq))
 #[2, 4, 6, 8, 10]

The filter function takes in a list and a rule, much like map, however it returns a subset of the original list by comparing each element against the boolean filtering rule.

seq = [1, 2, 3, 4, 5]
list(filter(lambda x: x > 2, seq))
# [3, 4, 5]

Arange and Linspace

For creating quick and easy Numpy arrays, look no further than the arange and linspace functions. Each one has their specific purpose, but the appeal here (instead of using range), is that they output NumPy arrays, which are typically easier to work with for data science.

Arange returns evenly spaced values within a given interval. Along with a starting and stopping point, you can also define a step size or data type if necessary. Note that the stopping point is a ‘cut-off’ value, so it will not be included in the array output.

# np.arange(start, stop, step)
np.arange(3, 7, 2)
# array([3, 5])

Linspace is very similar, but with a slight twist. Linspace returns evenly spaced numbers over a specified interval. So given a starting and stopping point, as well as a number of values, linspace will evenly space them out for you in a NumPy array. This is especially helpful for data visualizations and declaring axes when plotting.

# np.linspace(start, stop, num)
np.linspace(2.0, 3.0, num=5)
# array([ 2.0,  2.25,  2.5,  2.75, 3.0])

Download image from url

import requests
import os

def download_image(url, dir):
    if url.endswith(('.jpg', '.jpeg', '.png', '.gif')):
        img = requests.get(url).content
        file_name = os.path.basename(url)
        with open(f'{dir}{file_name}', 'wb') as f:
            f.write(img)

download_image('https://impshum.co.uk/red.png', './')

Create centered thumbnail from image

from PIL import Image

def create_thumbnail(infile, outfile, width, height):
    thumb = width, height
    img = Image.open(infile)
    width, height = img.size

    if width > height:
        delta = width - height
        left = int(delta / 2)
        upper = 0
        right = height + left
        lower = height
    else:
        delta = height - width
        left = 0
        upper = int(delta / 2)
        right = width
        lower = width + upper

    img = img.crop((left, upper, right, lower))
    img.thumbnail(thumb, Image.ANTIALIAS)
    img.save(outfile)

create_thumbnail('file.jpg', 'file_thumb.jpg', 300, 300)

Handle keyboard interrupt (Ctrl + C)

try:
    # DO STUFF HERE
except KeyboardInterrupt:
    print('stopped')
finally:
    # DO STUFF HERE
    print('Exiting')

Merge dictionaries

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

z = {**x, **y}