python-tricks.md

Python Tricks

Count the Lines of Code in Your Project Directory

pip install pygount
pygount --format=summary /path/to/your/directory

Virtual Environment

# sudo apt install python3-venv

# create a virtual environment
python3 -m venv env-name

# activate the venv
source env-name/bin/activate

# deactivate
deactivate

Upgrade Python

# if Python 3.8 is not on Ubuntu default repositories
sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt-get update
sudo apt list | grep python3.8

# install
sudo apt-get install python3.8

# add both the old and new Python to update-alternatives
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 2

# update the default option
sudo update-alternatives --config python3
python3 -V

# optional: install related libraries
sudo apt install python3.8-dev

Note: update-alternatives creates, removes, maintains and displays information about the symbolic links comprising the Debian alternatives system.

Install Packages on a Machine without Internet

On the machine with access to the Internet:

mkdir py-packages
pip download package-1 -d "py-packages"
pip download package-2 -d "py-packages"
tar cvfz py-packages.tgz .\py-packages

On the machine without access to the Internet:

tar xvfz .\py-packages.tgz
pip install py-packages\package-1.whl -f ./ --no-index

Note: You may need to install according to the dependency order between packages. Or you can add -no-deps to install the package.

Auto requirements.txt Generation

pip install pipreqs
pipreqs /path/to/project

You probably need to add Python binary file location to $PATH after installing pipreqs.

Interchange Values Between Two Variables

x, y = 10, 20
print(x, y)

x, y = y, x
print(x, y)

#1 (10, 20)
#2 (20, 10)

Chain of Comparison

n = 10
result = 1 < n < 20
print(result)

# True

result = 1 > n <= 9
print(result)

# False

If-Sels Assignment

x = 10 if (y == 9) else 20
x = (classA if y == 1 else classB)(param1, param2)
[m**2 if m > 10 else m**4 for m in range(50)]

List Assignment

testList = [1,2,3]
x, y, z = testList

print(x, y, z)

#-> 1 2 3

Printing Path of Imported Library

import threading
import socket

print(threading)
print(socket)

#1- <module 'threading' from '/usr/lib/python2.7/threading.py'>
#2- <module 'socket' from '/usr/lib/python2.7/socket.py'>

Temporary Variable for Last Expression

>>> 2 + 1
3
>>> _
3
>>> print _
3

Dictionary/Set Comprehension

testDict = {i: i * i for i in xrange(10)}
testSet = {i * 2 for i in xrange(10)}

print(testSet)
print(testDict)

#set([0, 2, 4, 6, 8, 10, 12, 14, 16, 18])
#{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}

Debugging Scripts - Set Breaking Points

import pdb
pdb.set_trace()

Inspect Objects

test = [1, 3, 5, 7]
print(dir(test))

Check Python Version At Runtime

import sys

# or you can use sys.version_info >= (3, 5)
if not hasattr(sys, "hexversion") or sys.hexversion != 50660080:
    print("Sorry, you aren't running on Python 3.5n")
    print("Please upgrade to 3.5.n")
    sys.exit(1)

# print Python version in a readable format.
print("Current Python version: ", sys.version)

Use Splat Operator to Unpack Parameters

def test(x, y, z):
    print(x, y, z)

testDict = {'x': 1, 'y': 2, 'z': 3}
testList = [10, 20, 30]

test(*testDict)
test(**testDict)
test(*testList)

#1-> x y z
#2-> 1 2 3
#3-> 10 20 30

Check Memory Usage for Variables

import sys

x=1
print(sys.getsizeof(x))

In Python 2.7, a 32-bit integer uses 24 bytes, while in Python 3.5, it uses 28 bytes.

Scope

Variable scope only applies at the function, module, and class levels. If you are in the same function/module/class, all variables defined will be available within that function/module/class, regardless of whether it was defined within a with, for, if, etc. block.

For example, this is valid.

for x in range(1):
    y = 1
print(y)

However, you must be careful since the variable defined within your code block might not actually be defined if the block is never entered, as in this case:

try:
    with open('filedoesnotexist', 'r') as file:
        pass
except:
    pass # just to emphasize point

print(file.mode)

Traceback (most recent call last):
  File "<pyshell#43>", line 1, in <module>
    file.mode
NameError: name 'file' is not defined