Python

Python zen

import this

Syntax cheatsheet

if blah == 0:
    print("0")
elif == 1:
    print("1")
else:
    print("else")

def printarg (arg):
    print(arg)

if __name__ == "__main__":
    main()

String formatting

user="world"

# Interpolated (3.6+)
print(f"hello {user}")
# New
print("hello {}".format(user))
# Old
print("hello %s" % user)

enums

from enum import Enum
class Color(Enum):
    red = 1
    green = 2
    blue = 3

Getopts

import getopt, sys

try:
    opts, args = getopt.getopt(sys.argv[1:], 'eq', ["session=", "server="])
except getopt.GetoptError, err:
    # print help information and exit:
    print str(err)
    sys.exit(1)
for opt, val in opts:
    if opt in ('-e'):
        extend = True
    elif opt in ('-q'):
        query = True
    elif opt in ('--session'):
        session = val
    elif opt in ('--server'):
        server = val

Opening a file

try:
    with open(filepath, "r") as file_handle:
        for line in file_handle:
           ...
    except IOError, err:
        print(err)

Remove duplicates from a list

the_list = list(set(the_list))

Profile a program

http://docs.python.org/library/profile.html

import profile
profile.run("main()")

Launch REPL when line is hit

import code
code.interact(local=locals())

Logging

import logging
logging.basicConfig(filename='myapp.log', level=logging.INFO)
logging.warning('%s before you %s', 'Look', 'leap!')

Named tuples (pseudo-classes)

import collections

Prisoner = collections.namedtuple('Prisoner', 'name rank serial')

hogan = Person(name='Hogan', age='Colonel', serial='1234')
lebeau = Person(name='Lebeau', age='Private', serial='8888')
print 'Name:', lebeau.name

for prisoner in [ hogan, lebeau ]:
    print '%s is a %d, serial: %s' % prisoner

Tab completion in Python shell

If you don’t have access to IPython,

import rlcompleter, readline
readline.parse_and_bind('tab: complete')

Tar a bunch of files

import tarfile
tar = tarfile.open("sample.tar", "w")
for name in ["foo", "bar", "quux"]:
    tar.add(name)
    tar.close()

Run a simple webserver

# Python 3
python -m http.server
# Python 2
python -m SimpleHTTPServer

Check Python version

if sys.hexversion >= 0x020502F0:
    # use some advanced feature
    ...
else:
    # use an alternative implementation or warn the user
    ...

bit	description
1-8	PY_MAJOR_VERSION (the 2 in 2.1.0a3)
9-16	PY_MINOR_VERSION (the 1 in 2.1.0a3)
17-24	PY_MICRO_VERSION (the 0 in 2.1.0a3)
25-28	PY_RELEASE_LEVEL (0xA for alpha, 0xB for beta, 0xC for release candidate and 0xF for final)
29-32	PY_RELEASE_SERIAL (the 3 in 2.1.0a3, zero for final releases)

Get Linux distribution

if sys.hexversion < 0x020600F0:
    platform.dist()
else:
    platform.linux_distribution()

Inspect the stack

import inspect
print " << ".join([i[3] for i in inspect.stack()])

Disassemble a code object

import dis
dis.dis(func)

http://akaptur.com/blog/2013/08/14/python-bytecode-fun-with-dis/

Create virtual environment

python3 -m venv path/to/new/env

platform

# Get machine type
platform.machine()
# Determine system type (e.g. Linux, Windows)
platform.system()
# Get Linux distro
platform.linux_distribution()

unittest

Up to 2.7, use unittest2 since it’s got more assert tests than unittest (they were all backported from the 2.7 release).

Running tests

python -m unittest discover -s project_directory -p '*_test.py'

Example

import random
import unittest

class TestSequenceFunctions(unittest.TestCase):

def setUp(self):
    self.seq = range(10)

def test_shuffle(self):
    # make sure the shuffled sequence does not lose any elements
    random.shuffle(self.seq)
    self.seq.sort()
    self.assertEqual(self.seq, range(10))

# should raise an exception for an immutable sequence
self.assertRaises(TypeError, random.shuffle, (1,2,3))

def test_choice(self):
    element = random.choice(self.seq)
    self.assertTrue(element in self.seq)

def test_sample(self):
    with self.assertRaises(ValueError):
        random.sample(self.seq, 20)
    for element in random.sample(self.seq, 5):
        self.assertTrue(element in self.seq)

if __name__ == '__main__':
    unittest.main()

Python 3 Upgrading

• http://python-future.org/futurize.html
• https://python-modernize.readthedocs.io/en/latest/

six

2to3

• https://docs.python.org/3/library/2to3.html

# Auto-correct files
2to3 -w whatever.py

caniusepython3

• https://caniusepython3.com/
• https://pypi.org/project/caniusepython3/

daemonizing

import sys, os
def daemonize(stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
try:
pid = os.fork()
if pid > 0:
sys.exit(0)
except OSError, e:
sys.stderr.write("fork #1 failed: (%d) %s\n" % (e.errno, e.strerror))

os.chdir("/")
os.umask(0)
os.setsid()

try:
pid = os.fork()
if pid > 0:
sys.exit(0)
except OSError, e:
sys.stderr.write("fork #2 failed: (%d) %s\n" % (e.errno, e.strerror))
sys.exit(1)

for f in sys.stdout, sys.stderr: f.flush()
si = file(stdin, 'r')
so = file(stdout, 'a+')
se = file(stderr, 'a+', 0)
os.dup2(si.fileno(), sys.stdin.fileno())
os.dup2(so.fileno(), sys.stdout.fileno())
os.dup2(se.fileno(), sys.stdout.fileno())

email

Run a simple SMTP server

python -m smtpd -n -c DebuggingServer localhost:1025

Send email

import smtplib
mail_server = 'localhost'
mail_server_port = 25

from_addr = 'sender@example.com'
to_addr = 'receiver@example.com'

from_header = 'From: %s\r\n' %from_addr
to_header = 'To: %s\r\n\r\n' %to_addr
subject_header = 'Subject: nothing interesting'

body = 'This is a not-very-interesting email'

email_message = '%s\n%s\n%s\n\n%s' % (from_header, to_header, subject_header, body)

s = smtplib.SMTP(mail_server, mail_server_port)
s.sendmail(from_addr, to_addr, email_message)
s.quit()

Retrieve mail with imaplib

import imaplib

username = 'name'
password = 'pass'

mail_server = 'mail_server'

i = imaplib.IMAP4_SSL(mail_server)
i.login(username, password)
i.select('INBOX')

for msg in i.search(None, 'ALL')[1][0].split():
    print msg
outf = open('%s.eml' % msg, 'w')
outf.write(i.fetch(msg, '(RFC822)')[1][0][1])
outf.close()
i.logout()

ftplib

from ftplib import FTP
server = FTP('ftp.example.com')
server.login()

HTTPServer

Basic HTTP server

from BaseHTTPServer import HTTPServer, BaseHTTPRequestHandler
from SocketServer import ThreadingMixIn

class RedirectHandler(BaseHTTPRequestHandler):
def do_HEAD(self):
self.send_response(200)
self.send_header("Content-type", "text/html")
self.end_headers()

def do_GET(self):
self.send_response(200)
self.send_header("Content-type", "text/html")
self.end_headers()
self.wfile.write("<html><head><title>Example</title></head>")
self.wfile.write("<body>%s</body></html>" % self.path)

class ThreadedHTTPServer(ThreadingMixIn, HTTPServer):
"""
Thread the responses
"""
if __name__ == '__main__':
host = "0.0.0.0"
port = 8080
try:
httpd = ThreadedHTTPServer((host, port), RedirectHandler)
httpd.serve_forever()
except:
print "Could not bind to host %s port %i\n" % (host, port)

subprocess with Popen

pipe = subprocess.Popen(args_list, shell=True, stdout=subprocess.PIPE)
output - pipe.communicate()[0]

pdb

cmd	desc
h	help
w	(where) stacktrace
l	list source
s	step
n	next
d	down frame
b	breakpoint
tb	temporary breakpoint

CLI

python -m pdb file.py

Invoke debugger when line is hit

# Python 3.7+
breakpoint()

# Python < 3.7
import pdb
pdb.set_trace()

pydoc

# Read module docs in terminal
pydoc $MODULE

# Start documentation server, open browser
pydoc -b

# Start documentation server at port 1234
pydoc -p 1234

Python - D-Bus

https://dbus.freedesktop.org/doc/dbus-python/ https://pypi.org/project/dbus-python/

Connecting to D-Bus

import dbus

bus = dbus.SystemBus()
try:
    thing = bus.get_object('com.tech.Thing', '/com/tech/Thing')
except:
    print("Oh snap, couldn't connect")

Tkinter

from Tkinter import *

class MyDialog:
    def __init__(self, parent):
        self.top = Frame(parent)
        self.top.pack()

        Label(self.top, text="Value").pack()

        self.e = Entry(self.top)
        self.e.pack(padx=5)

        b = Button(self.top, text="OK", command=self.ok)
        b.pack(pady=5)

    def ok(self):
        print "value is", self.e.get()
        self.top.destroy()

root = Tk()
root.title("Some title!")
d = MyDialog(root)
root.wait_window(d.top)

XMLRPC

Connecting to an XMLRPC server (Python 2)

import xmlrpclib

server = '<http://blah.com/XMLRPC>'
svr = xmlrpclib.Server(server)

session_id = '2'
params = { "session_id" : int(session_id)}
response = svr.foo.bar(params)

Packaging

PyPI

• List of classifiers

Uploading a new package

python setup.py register
python setup.py sdist upload

pip

Create a package list

pip freeze > requirements.txt

Upgrade package

pip install -U package

Install a package list

pip install -r requirements.txt

Dreamhost pip3 usage

virtualenv -p python3 env
. env/bin/activate
pip3 install #...

Conda

https://www.anaconda.com/products/individual

Fedora:

sudo dnf install conda

# Create an env
conda create -n ENVNAME
# Activate the env
conda activate ENVNAME

Third party

Django

Start project

django-admin.py startproject project_name

Start demo server

python manage.py runserver

Check for construction errors in models

python manage.py validate

Put models in database

python manage.py syncdb

Shell

python manage.py shell

Permanent redirect

in urls.py’s urlpatterns

url(r'^$', lambda x: HttpResponsePermanentRedirect('/n')

taggit

Installing in a project

• pip install django-taggit
• add ‘taggit’ to ~INSTALLED_APPS
• ./manage.py syncdb

from django.db import models
from taggit.managers import TaggableManager

class Thing(models.Model):
    # ... fields here

tags = TaggableManager()

South

Installing in a project

• pip install South
• add ‘south’ to ~INSTALLED_APPS
• ./manage.py syncdb

Adding South to an existing project

• add ‘south’ to ~INSTALLED_APPS
• ./manage.py syncdb
• ./manage.py convert_to_south myapp

Initial migrate schema creation

./manage.py schemamigration app --initial
./manage.py migrate app

Change schema

./manage.py schemamigration app --auto
./manage.py migrate app

Enabling admin mode

• Uncomment “django.contrib.admin” in the ~INSTALLED_APPS setting.
• ./manage.py syncdb
• Uncomment admin lines in urls.py
• Create admin.py in your app:

from django.contrib import admin
from app.models import Thing

admin.site.register(Thing)

Reset admin password w/o password reset

./manage.py shell

from django.contrib.auth.models import User
users = User.objects.all()
users
users[0].set_password('whatever');
users[0].save()

Tornado

Using SSL:

server = tornado.httpserver.HTTPServer(application, ssl_options={
"certfile": "/path/to/ssl/cert.crt",
"keyfile": "/path/to/ssl/tornado_key.pem",
})
server.bind(port)
server.start(0)
tornado.ioloop.IOLoop.instance().start()

autopep8

Type-checking

Pyre

Requires Python 3.6+ and watchman

# Install pyre
pip install pyre-check
pyre init
pyre

mypy

PyGTK

import pygtk
pygtk.require('2.0')
import gtk

class PyGUI():
    def __init__(self):
        try:
            builder = gtk.Builder()
            builder.add_from_file("gui.glade")
        except:
            self.stderr("Failed to load Glade template file")
            sys.exit(1)

        self.main_window = builder.get_object("main_window")

        builder.connect_signals(self)

    def main(self):
        self.main_window.show()
        gtk.main()