Difference between revisions of "Short Notes on Python"

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== iPython fails with <tt>NameError: name 'sys' is not defined</tt> on autocompletion ==
== iPython fails with ''NameError: name 'sys' is not defined'' on autocompletion ==
This is due to new version of <tt>jedi</tt> module not being compatible with iPython.
This is due to new version of <tt>jedi</tt> module not being compatible with iPython.

Revision as of 16:14, 17 May 2021


Date, Time, Datetime

d: datetime.date = ...
t: datetime.time = ...
dt: datetime.datetime = ...

dt.date() -> datetime.date
dt.time() -> datetime.time
datetime.datetime.combine(d, t) -> datetime.datetime
in particular: datetime.datetime.combine(d, datetime.time(0, 0, 0)) -> start of date `d` as datetime

strftime() Formatting

Examples for Thu Dec 3, 2020, 01:23:45.678 - datetime.datetime(2020, 12, 3, 1, 23, 45, 678000)

%y       - year (20)
%Y       - year (2020)
%m / %-m - month zero-/un- padded (12 / 12)
%d / %-d - day zero-/un- padded (03 / 3)
%H / %-H - hour zero-/un- padded, 24-hour clock (01 / 1)
%I / %-I - hour zero-/un- padded, 12-hour clock (01 / 1)
%p       - time of day (AM)
%M / %-M - minute zero-/un- padded (23 / 23)
%S / %-S - second zero-/un- padded (45 / 45)
%f       - microsecond (678000)
%z       - timezone as +HHMM or -HHMM, empty if object is naive
%Z       - timezone name, empty if object is naive
%j / %-j - day of year zero-/un- padded (338 / 338)
%a / %A  - weekday name, abb. / full (Thu / Thursday)
%w       - weekday as decimal, 0-Sunday, 6-Saturday (4)
%U / %W  - week number, week starting on Sunday / Monday (48 / 48)
%b / %B  - month name, abb. / full (Dec / December)
%c       - locale appropriate date and time representation (Thu Dec  3 01:23:45 2020)
%x       - locale appropriate date representation (12/03/20)
%X       - locale appropriate time representation (01:23:45)


For me, the following does a good job getting memory usage (in kB) on Linux:

import resource
print resource.getrusage(resource.RUSAGE_SELF).ru_maxrss

Since resource is standard package, it should work on Windows too, but I don't know if it does, or what units are used if it works.

Importing Files

If you need to import a file '../mylib/commons.py', you can use the following snippet:

import sys, os
fld = os.path.realpath(os.path.abspath(os.path.join('..', 'mylib')))
if fld not in sys.path:
    sys.path.insert(0, fld)
import commons

# use your commons.py module now...

Merging Dictionaries

Below are few solutions, the first one works for any list of dictionaries, the rest is just for 2 dict's, with the last 2 suitable for inlining:

# as a generic function that merges list of dict's
def merge_dicts(dicts: list) -> dict:
    res = dict()
    for d in dicts:
    return res

# if you're merging known number of dict's:

def merge_two_1(a: dict, b: dict) -> dict:
    return dict(a, **b)

# python 3.5+, the fastest of the lot
def merge_two_1(a: dict, b: dict) -> dict:
    return {**a, **b}

uWSGI, nginx, Flask

  • install uwsgi (incl. uwsgi python plugin), python flask, and nginx,

Setting Up uWSGI

  • create main.py file that will hold the server logic, for instance:
from flask import Flask

app = Flask(__name__)

def hello():
    return "hello there!"
  • create uwsgi config file, wsgi.ini (minimal version here; read uwsgi docs for head-spinning array of configurables):
module          = main:app
master          = true
processes       = 5
socket          =
protocol        = http
plugin          = python
  • run uwsgi
uwsgi --ini wsgi.ini

Adding nginx Layer

  • remove the "protocol" directive from wsgi.ini, and add "die-on-term":
module          = main:app
master          = true
processes       = 5
socket          =
plugin          = python
die-on-term     = true
  • add a new vhost to nginx - /etc/nginx/sites-available/app.nginx:
server {
    listen 80;
    server_name my.awesome.domain;
    location / {
        include uwsgi_params;
    • communication through socket is also possible (see socket, chmod-socket, vacuum and other directives for uWSGI)
    • of course, create link in /etc/nginx/sites-enabled/, and restart nginx,

Run uWSGI daemon on boot - supervisor

  • install supervisor
apt-get install supervisor
  • add/edit /etc/supervisord.conf with content like this:

loglevel=info ; (others: warn,debug,trace)


[program:your app]
command=/usr/bin/uwsgi --ini wsgi.ini

Run uWSGI daemon on boot - systemd

  • create systemd file for uWSGI, /etc/systemd/system/uwsgi-app.service:
Description=Job that runs the uWSGI app

ExecStart=/usr/bin/uwsgi --ini wsgi.ini


Then you can start and stop the uwsgi service using:

# systemctl start uwsgi-app.service
# systemctl stop uwsgi-app.service

Once you're happy with the settings, enable the daemon to be run on boot:

# systemctl enable uwsgi-app.service


needs a bit of refreshment and updating...

based on.

Decorators are simple and expressive way to modify function without editing the source of the function itself. Or, the other way around, to modify multiple functions in the same way, without code duplication (DRY).

Decorators can be spotted in the code by starting with @ character. Decorator is a function (or class) that can do some additional work before or after the call to the decorated function. It can even call the decorated function multiple times, or not at all.

Decorators can be implemented as closures (my fave), or as classes; the following approaches are equivalent:

# using a function (closure) as a decorator
def beforeAndAfter(f):
    def decorated_fn():
        print("Before", f.__name__)
        print("After", f.__name__)
    return decorated_fn

def func():
    print "func() is in da' house!"

# using a class as a decorator
class beforeAndAfter(object):

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

    def __call__(self):
        print("Before", self.f.__name__)
        print("After", self.f.__name__)

def func():
    print "func() is in da' house!"


Note that in functional decorator, anything outside the body of decorated_fn() is equivalent to content of the __init__() constructor of the class-based decorator. This code is run during the initialization, only once for each decorated function, regardless of whether the decorated function is ever called in the code - you should avoid any heavy lifting there.

Decorators can also accept arguments, and (obviously should) forward arguments to the decorated function:

def beforeAndAfter(p1, p2):
    def wrap(f):
        def wrapped_f(*args, **kwargs):
            print "Decorator arguments:", p1, p2
            print("Before", f.__name__)
            f(*args, **kwargs)
            print("After", f.__name__)
        return wrapped_f
    return wrap

@beforeAndAfter("hello", "world")
def func(a, b=2):
    print "func() is in da' house,", a, b



I prefer installing virtualenv tool through pip, to make sure those are in sync version-wise.

# create a new venv
# it is better to keep venv's in some separate folder, not to pollute your project folder
$ virtualenv ~/.virtualenvs/my-new-env

# "log into" your venv; success can be seen by your command line being prefixed by venv name
$ source ~/.virtualenvs/my-new-env/bin/activate

# now you can install anything you need, tucked away in your venv:
(my-new-env) $ pip install -r requirements.txt

# when done, just deactivate
(my-new-env) $ deactivate


Select, Where, Group


        Table.column1: 10,
        Table.column2: Table.column2 + 50

Scalar Values

min, max, ...

from sqlalchemy import func

max_value = session.query(sqlalchemy.func.max(Table.column)).scalar()

# or, for multiple, just use
session.query(sqlalchemy.func.min(Table.column), sqlalchemy.func.max(Table.column)).first()

select count(*)


Make SQLAlchemy Read-Only

All write operations in SQLAlchemy pass through flush() method of your session. Just monkey-path it to do nothing!

engine = create_engine("connection string")
Session = sessionmaker(bind=engine, autoflush=False, autocommit=False)
session = Session()
session.flush = lambda *args,**kwargs: None

Making Session Self-Recoverable

Some DB's, most prominently Postgres, do not recover well from errors (e.g. Pg just keeps saying that there's error in current transaction). One way is to rollback the transaction on "any" error:

from contextlib import contextmanager
from typing import ContextManager
from sqlalchemy.orm.session import Session

def db_session() -> ContextManager[Session]:
    session = init()    # get the session any way you like
        yield session

def use_db():
    with db_session() as session:
        # use session object

Show Create Statement for Your Model, including Indexes

from sqlalchemy.schema import CreateTable, CreateIndex
from sqlalchemy.dialects import postgresql

def print_create_statement(model):
    for index in model.__table__.indexes:

List Tables and Their Columns

import sqlalchemy as sqla

db_conn_str = "..your connection string.."
engine = sqla.create_engine(db_conn_str)
inspector = sqla.inspect(engine)
schemas = inspector.get_schema_names()

for schema in schemas:
    print("schema: {}".format(schema))
    for table_name in inspector.get_table_names(schema=schema):
        cols = ['{} ({})'.format(col['name'], col['type']) for col in inspector.get_columns(table_name, schema=schema)]
        print("Table: {} ({})".format(table_name, ', '.join(cols)))


Creating subqueries is pretty straightforward in SQLAlchemy.

To emit

SELECT sub.c1, sub.c2
FROM (SELECT x as c1, y as c2 FROM table) AS sub

you define the sub and then query on it as usual, with the exception of bit more explicit access to columns:

sub = session.query(table.x.label('c1'),

res = session.query(sub.c.c1, sub.c.c2).\

iPython fails with NameError: name 'sys' is not defined on autocompletion

This is due to new version of jedi module not being compatible with iPython.

Just downgrade it to pre-0.18 version, using

pip install --upgrade 'jedi<0.18.0'