• exception handling in python (tutswiki.com)
  • very easy to follow; high information density; well thought out examples
• Online python compiler (programiz.com)
  • can be useful while doing live coding interviews.
• Facts and myths about Python names and values (nedbatchelder.com) - well explained.

• os.path.basename - https://docs.python.org/3/library/os.path.html#os.path.basename
• continue statement - https://docs.python.org/3/reference/simple_stmts.html#the-continue-statement

Sample snippet:

day = 'foo'
valid_days = ['sun', 'mon', 'tue']
assert day in valid_days, '{} is not a valid day. '\
                          'It should be one of {}'.format(day, valid_days)

Sample output:

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-4-70d26d6f2c6d> in <module>
----> 1 assert day in valid_days, '{} is not a valid day. '\
      2                           'It should be one of {}'.format(day, valid_days)

AssertionError: foo is not a valid day. It should be one of ['sun', 'mon', 'tue']

In Python, the mod operator returns a number with the same sign as the second argument:

>>> 10%3
1
>>> (-10)%3
2
>>> 10%(-3)
-2
>>> (-10)%(-3)
-1

>>> import sys; print(sys.version)
3.11.9 (main, Apr 19 2024, 16:48:06) [GCC 11.2.0]

tags | module path

one way:

import importlib.util
importlib.util.find_spec("foo")

Alternative:

import imp
imp.find_module("foo")

Note:

• The imp module is deprecated in favour of importlib. So use the first approach whenever possible.

my_function.__name__

Ref:- https://stackoverflow.com/questions/251464/how-to-get-a-function-name-as-a-string

Task: If input file is empty, write an empty output file

import os
if os.stat(input_file).st_size == 0:
    logger.info('Input file is empty.')
    logger.info('Writing an empty output file.')
    with open(output_file, 'w') as fp:
        pass

• https://docs.python.org/3/library/configparser.html#configparser.ConfigParser.defaults - start from here to see the list of functions available
• use extension .cfg for config files.
  • based on: https://docs.python.org/3/library/configparser.html → search for .cfg
  • another choice is .ini . But I prefer .cfg over .ini
  • more info on INI: https://en.wikipedia.org/wiki/INI_file

• archives -https://mail.python.org/pipermail/python-list/
• list info - https://mail.python.org/mailman/listinfo/python-list

• operator precedence - https://docs.python.org/3/reference/expressions.html#operator-precedence

• Classes - https://docs.python.org/3/tutorial/classes.html
• https://docs.python.org/3/tutorial/index.html

• https://python-release-cycle.glitch.me/ - useful to check when a Python version was released and when it would be EOL.

In [1]: 
isinstance(True, int)
Out[1]: 
True

In [2]: 
type(True) is int
Out[2]: 
False

Using Python 3.9.5

with open('filename.txt', 'w') as fh:
  fh.write('\n')

Do not use os.linesep as a line terminator when writing files opened in text mode (the default); use a single '\n' instead, on all platforms.

Ref:- https://docs.python.org/3/library/os.html#os.linesep

See also:- https://stackoverflow.com/questions/4025760/python-file-write-creating-extra-carriage-return

pip install pylint

pylint module_name.py

find . -name "*.py" | xargs pylint

% python
Python 3.11.4 (main, Jul  5 2023, 13:45:01) [GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> isinstance(12.0, float)
True
>>> isinstance(12.0, int)
False
>>> isinstance(12, float)
False

% python
Python 3.11.4 (main, Jul  5 2023, 13:45:01) [GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> isinstance(12, int)
True
>>> isinstance(12, float)
False
>>> isinstance(12.0, int)
False

$ python
Python 3.9.17 (main, Jul  5 2023, 20:47:11) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> 4 == True
False
>>> 1 == True
True

Q: In Python 3 < 4 == True evaluates to False.

$ python
Python 3.9.17 (main, Jul  5 2023, 20:47:11) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> 3 < 4 == True
False

Why?

A: The < and == are comparison operators. If a, b, c, …, y, z are expressions and op1, op2, …, opN are comparison operators, then in python, a op1 b op2 c … y opN z is equivalent to a op1 b and b op2 c and … y opN z, except that each expression is evaluated at most once.

So 3 < 4 == True is equivalent to 3 < 4 and 4 == True which evaluates to True and False which turns out to be False.

Ref:-

• https://docs.python.org/3/reference/expressions.html#comparisons - describes the above rule and also gives a list of comparison operators

"<" | ">" | "==" | ">=" | "<=" | "!="
| "is" ["not"] | ["not"] "in"

Ref:- https://docs.python.org/3/reference/expressions.html#comparisons

 % python
>>> from math import comb
>>> comb(10, 3)
120

>>> import sys
>>> sys.version
'3.9.16 (main, May 15 2023, 23:46:34) \n[GCC 11.2.0]'

See also:

• https://stackoverflow.com/questions/4941753/is-there-a-math-ncr-function-in-python - contains recipes for older versions of python

• get urls in a url
• find depth

What is the difference between os.getenv vs os.environ.get?

None. I prefer to use os.getenv as it is less number of characters to type.

To experiment, use difference between os.getenv vs. os.environ.get.ipynb (github.com/KamarajuKusumanchi).

Sample usage from https://github.com/dpguthrie/yahooquery/blob/master/yahooquery/base.py#LL926C1-L936C1

    def __init__(self, **kwargs):
        ...
        username = os.getenv("YF_USERNAME") or kwargs.get("username")
        password = os.getenv("YF_PASSWORD") or kwargs.get("password")
        if username and password:
            self.login(username, password)

Say we have two projects cloned into two directories - github/project1 and github/project2. To import stuff from project1 into a file in project2

import os
import sys

# Note: os.path.dirname(__file__) can give an empty string,
# use os.path.dirname(os.path.abspath(__file__)) instead.
# Ref:- See Dmitry Trofimov's comment in https://stackoverflow.com/a/918178/6305733
dirname = os.path.dirname(os.path.abspath(__file__))

# Change the relative path as per your needs
newdir = os.path.abspath(os.path.join(dirname, '../project1'))

if newdir not in sys.path:
    sys.path.insert(0, newdir)

tags | import from many levels up, import from another directory, relative directory name, path relative to current file

import sys
if sys.version_info[:2] < (3, 7):
    raise RuntimeError('Requires Python 3.7 or greater')

from traceback import format_exc

try:
    # my code
except Exception as exc:
    print 'caught exception while doing foo'
    print format_exc()
    print exc

from traceback import format_exc

try:
    # my code
except Exception as exc:
    logger.error('Caught exception while doing stuff.')
    logger.error(format_exc())
    logger.error(exc)

When the code is executed in pycharm and the exception is encountered, the traceback is printed in both pycharm window and in the file.

To test the above code, you can raise a simple exception using

raise ValueError("test exception handling.")

tags | sleep for N seconds

from time import sleep
sleep(10)

Ref:- https://realpython.com/python-sleep/ - well written article that talks about how you might use the sleep function in real life. Worth reading end-to-end once.

Given

$ cat -A great.txt
kamaraju$
kamaraj $
kamara  $
kamar   $
kama    $
kam     $
ka      $
k       $

where the $ denotes the end of line.

$ du -b great.txt
72      great.txt

$ wc great.txt
 8  8 72 great.txt

To read it

In [3]:
file = 'great.txt'
with open(file) as fh:
    contents = [line.rstrip('\n') for line in fh]
print(contents)
['kamaraju', 'kamaraj ', 'kamara  ', 'kamar   ', 'kama    ', 'kam     ', 'ka      ', 'k       ']

In [4]:
len(contents)
Out[4]:
8

Ref:- https://stackoverflow.com/questions/15233340/getting-rid-of-n-when-using-readlines

You can also do it using readlines.

In [5]:
file = 'great.txt'
with open(file) as fh:
    lines = fh.readlines()
    lines = [line.rstrip('\n') for line in lines]
print(lines)
['kamaraju', 'kamaraj ', 'kamara  ', 'kamar   ', 'kama    ', 'kam     ', 'ka      ', 'k       ']

tags | file readlines strip newline

Another approach is to use read() with splitlines().

In [6]:
file = 'great.txt'
with open(file) as fh:
    lines = fh.read().splitlines()
print(lines)
['kamaraju', 'kamaraj ', 'kamara  ', 'kamar   ', 'kama    ', 'kam     ', 'ka      ', 'k       ']

import sys
print(sys.executable)

For example

$ python
Python 3.11.5 | packaged by Anaconda, Inc. | (main, Sep 11 2023, 13:26:23) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> print(sys.executable)
C:\Users\kkusuman\AppData\Local\conda\conda\envs\py311\python.exe

import os
home = os.path.expanduser("~")

Ref:- https://stackoverflow.com/questions/4028904/what-is-a-cross-platform-way-to-get-the-home-directory

import os
home = os.path.expanduser("~")
path = os.path.join(home, "x", "data.txt")

The expression round(n, r) rounds floating-point expression $n$ to the $10^{-r}$ decimal digit; for example, round(n, -2) rounds floating-point value $n$ to the hundreds place $(10^2)$. Similarly, round(n, 3) rounds floating-point value n to the thousandths place $(10^{-3})$.

$ cat junk51.py
letters = ["a", "b", "c"]
x = "1"
assert x in letters, "{} is not a valid letter. It should be one of {}".format(
    x, letters
)

$ python junk51.py
Traceback (most recent call last):
  File "C:\Users\kkusuman\x\junk51.py", line 3, in <module>
    assert x in letters, "{} is not a valid letter. It should be one of {}".format(
AssertionError: 1 is not a valid letter. It should be one of ['a', 'b', 'c']

• https://stackoverflow.com/a/49271549
  • refers to | http://web.archive.org/web/20180416235121/http://ihorbobak.com/index.php/2018/02/22/python-process-memory-profiling/

tags | passing function, function as value in dictionary, dispatch

• https://softwareengineering.stackexchange.com/questions/182093/why-store-a-function-inside-a-python-dictionary/182095

• https://stackoverflow.com/questions/44834/what-does-all-mean-in-python - good explanation by Alec Thomas
• https://docs.python.org/3/tutorial/modules.html#importing-from-a-package - link to documentation; referenced in the above stackoverflow link.

• https://superuser.com/questions/1633073/why-are-tar-xz-files-15x-smaller-when-using-pythons-tar-library-compared-to-mac - Why are tar.xz files 15x smaller when using Python's tar library compared to macOS tar?

• os.path.splitext(path) - Get the extension of a path
  • https://docs.python.org/3/library/os.path.html#os.path.splitext