Python

Updated (95990c1)

Summary

Main page for all things Python. Other pages cover specific topics, such as:

Installation

Anaconda

An option to install Python is to use Anaconda1. Download the appropriate installation file from https://www.anaconda.com/products/individual. And then run the installer with, e.g., sh ./Anaconda3-2021.11-Linux-x86_64.sh. The installation would typically be under $HOME/anaconda3. There is a page dedicated to configuring and using [Anaconda].

Language changes

In 2021, the Python steering council accepted the proposal to add a pattern-matching primitive to the language. The proposal consists of PEP634 along with PEP635 and PEP636.

pip

No binary install

To install a package from source with pip specify

pip install --no-binary $PACKAGE

Since requirements files are passed as command-line options to pip, you can also specify it as

some-package
--no-binary
another-package

Additionaly this will also work on setup.py’s install_requires. For instance:

setup(
    install_requires=[
        "some-package==0.0.1 --no-binary"
])

Modules

Relative import in Python 3

If a relative import is present inside a Python 3 file (e.g. file1) inside a module (e.g. mymod), say

from .foo import bar

We will encounter the error

ImportError: attempted relative import with no known parent package

A possible solution is to include the following in your module’s __init__.py:

import os, sys

sys.path.append(os.path.dirname(os.path.realpath(__file__)))

Ternary operator

Ternary operators help reduce the amount of very small if-else blocks. Python does not have a ternary operator like other languages. However, conditionals can be used to the same effect:

y = 7
x = 0 if (y == 1) else 1
print(x)

1

for … else

for-else blocks allow to capture if a condition was met inside a for-loop. For instance, consider the following for-loop:

locations = ['a', 'b', 'c', 'd', 'f']
treasure = False
for location in locations:
   if location == 'x':
       treasure = True
       break
if not treasure:
   print("X marks the spot, but not found")

X marks the spot, but not found

We can simplify the above logic using a for-else loop:

for location in locations:
   if location == 'x':
       break
else:
   print("X marks the spot, but not found")

X marks the spot, but not found

Boolean unravelling

and

unravelling the and boolean operator. The operation can be rewritten as the function u_and:

def u_and(a, b):
   result = a
   if a:
      result = b
   return result

For instance:

a = True ; b = None
print(a and b, u_and(a, b))
a = True ; b = True
print(a and b, u_and(a, b))
a = False ; b = True
print(a and b, u_and(a, b))

None None
True True
False False

or

On the other hand, or cand be unravelled as:

def u_or(a, b):
   result = a
   if not a:
      result = b
   return result

As an example:

a = True ; b = None
print(a or b, u_or(a, b))
a = True ; b = True
print(a or b, u_or(a, b))
a = False ; b = True
print(a or b, u_or(a, b))

True True
True True
True True

The many faces of print

Concatenating arguments

var1 = "Foo"
var2 = "Bar"
print("I am ", var1, " not ", var2)

I am  Foo  not  Bar

It is also possible to use separators by using the sep argument:

var1 = "Foo"
var2 = "Bar"
print("I am", var1, "not", var2, sep="!")

I am!Foo!not!Bar

String termination

The end argument allows to specify the suffix of the whole string.

print("This is on radio", end=" (over)")

This is on radio (over)

Filesystem operations

Get home directory

For Python +3.5:

from pathlib import Path

home = str(Path.home())
print(home)

/Users/rui

List files recursively

For Python +3.5, use glob:

import glob

# root_dir with trailing slash (i.e. /root/dir/)
root_dir = "./"
for filename in glob.iglob(root_dir + '**/*.md', recursive=True):
     print(filename[:-3])

./Python
./Python code style
./Python Abstract classes
./New Features in Python 3.9
./Understanding Decorators in Python

Date operations

Offset-aware operations

Let’s say you have a date without timezone (offset naive), for instance:

from datetime import datetime, timezone

ts = datetime.now().replace(tzinfo=None)
print(ts)

2022-08-07 14:08:13.498348

And you want to calculate the $\delta$ with a datetime which has a time (offset aware). We’ll get an error.

try:
   delta = datetime.now(timezone.utc) - ts
except TypeError as error:
   print(error)

can't subtract offset-naive and offset-aware datetimes

The solution is to add a timezone to the offset naive date. For instance:

ts = datetime.now(timezone.utc)

delta = datetime.now(timezone.utc) - ts
delta

datetime.timedelta(microseconds=55)

Strings

Packaging

PyOxidizer

An alternative to package Python applications is PyOxidizer.

PyOxidizer is often used to generate binaries embedding a Python interpreter and a custom Python application. However, its configuration files support additional functionality, such as the ability to produce Windows

  1. https://www.anaconda.com/ ↩︎