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python notes

Defaultdict usage

from collections import defaultdict  
x = defaultdict(str)  
x['y'] # => ''  

Decode bytes to utf8 string


It’s ok to define a var in a catch block

# Do not need to define x here  
    raise ValueError("broken precondition")  
    x = 1  
x += 1  # => 2  

Huh, same as ruby

  raise 'bad'  
  x = 1  
x + 1  # => 2  

With js I need to define x ahead of time:

let x;  
try {  
  throw Error("Bad");  
} catch {  
  let x = 1;  
x + 1;  // => 2  

Run pytest with verbose output

pytest -svv  

Set a breakpoint

In modern versions of python breakpoint() will suffice.
No need to import ‘pdb’ explicitly.

TODO: This file needs to be formatted.

Also see ~/notes/pip.md
Also see ~/notes/pyenv.md

(python, inspiration, live coding, David Beazley, performance, concurrency, GIL, coroutines)
Source: https://news.ycombinator.com/item?id=36785005

(python simple http server, SimpleHTTPServer, python 3)
Serve current directory at http://localhost:8000
python -m http.server

(python, repl, save file)
After prototyping in the repl, dump all code I’ve entered into the repl:
%save my_file.py
So awesome!

(python, debugging)
Examine MRO with:
class Foo
class Bar:
class Baz(Foo, Bar):

(python, gotchas)

(pdb, command list)
unt is helpful to get out of a loop
Also use up and down

(ipython, mypy)
Check mypy types in the repl with:
pip install mypy_ipython
%load_ext mypy_ipython
… do some prototyping

(pdb, pytest)
run tests with –pdb to drop you into a repl on test failure

(pytest, mocking)
Use mock.mock_calls to get a list of all calls sent to a mock.
Or mock.call_args_list. I’m not sure what the difference is.

(print fstring, debug)
x = ‘hello’
print(f’{x = }’)

Also see alternatives ‘ice cream’ and ‘q’: https://github.com/zestyping/q

Use bt to print frames, then
f [number]
to jump to one

(python modify path)
PYTHONPATH=“./:$PYTHONPATH” python path/to/script.py

(read unit.xml files)
Install https://github.com/lukejpreston/xunit-viewer
pytest –junit-xml=build/unit.xml
xunit-viewer -r build/unit.xml

(python, pypi, list versions, pip list versions, hack)
pip install my-package==

(python zen of python)
import this

(python testing sibling library)
cd ~/dev/my_app
source .env/bin/activate
pip install -e ~/dev/my_lib

(python path, pythonpath, modify path, run from vim)

(files, naming, modules)
.py files can be imported as a module, but only if they do not contain hyphens in the name
bad: from bad-thing import foo
good: from good_thing import foo

(bdist, sdist)
See ~/dev/snippets/python/example_package/README.md and the associated
package layout

sdist is a source distribution, build with:
python setup.py sdist
Build wheel distribution with:
python setup.py sdist bdist_wheel

Note that .whl files can be unzipped with unzip

The distributions are saved in the relative dist/ directory. Untar the source
distribution or unzip the .whl distribution to debug.

To include a non-python file in the wheel, the setting include_package_data
must be True in setup.py AND an include statement must be present in

Wheel is the more modern form of egg: https://packaging.python.org/discussions/wheel-vs-egg/
> Wheel is currently considered the standard for built and binary packaging for Python.

Good slides here: https://blog.ionelmc.ro/presentations/packaging/#slide:1

(debugging, python, repl)
Pickle is fully specified to reconstruct types
Insert breakpoint in problematic process, pickle.dump to file
pickle.load in script with debugging aids

(help, repl)
Use help(sorted) to see docstring of stdlib sorted function
Works with modules:
import pickle

(mypy, readme)

(python get size of object, memory, ram, size in bytes)
import sys
See ~/notes/random_notes_worth_keeping.txt search ‘memory usage’

(module not found)
Seeing: ModuleNotFoundError: No module named ‘.
Fix: Add an init.py file into

(pip install git branch)
pip install git+https://github.com//@

(python and &, numpy, vectorized and)

(python2 to python3, migration, equivalence)

(python mocking)
Most articles I’ve found on python mocking have been junk. Collection of easy to follow tips:

(environment variables, env variables, python, virtual env)
- In a virtual environment, the path to env is stored at $VIRTUAL_ENV
- To add additional env variables, add to the bottom of .env/bin/activate, and
also unset inside the deactivate function

(find where package is located, installed)
$ python -v
> import

(python debugging, pdb)
Install pdb++ https://pypi.org/project/pdbpp/
pip install pdbpp
Then at the debugger prompt, type ‘sticky’ for a much better debugging experience

(python debugging, pdb, multiline)
type ‘interact’ at the pdb prompt

(virtual environment, virtual env, python2)
$ /usr/local/Cellar/python@2/2.7.15_2/bin/pip2.7 install virtualenv
$ python2.7 /usr/local/lib/python2.7/site-packages/virtualenv.py .env_2_7
$ source .env_2_7/bin/activate

(virtualenv, system python, python 2.7)
sudo /usr/bin/easy_install-2.7 pip
/usr/bin/python2.7 -m pip install virtualenv
/usr/bin/python2.7 -m virtualenv .env
source .env/bin/activate
pip install -r requirements.txt

(pdb, list, display, source, return to breakpoint)
list .

(ipython enter newline)
It’s tricky to add code to a previous code block after up-arrowing to it. To insert a newline, use:
ctlr+o+n or ctrl+q+j

(ipython read file, ipython load, ipython load script)
%load filename.py

(ipython, pdb)
Use pp to pretty print objects

(pytest hides prints, show print, pytest)
pytest -s to show print statements when you run tests
very confusing that the default setting eats print statements
pytest -s -vv to show verbose difference

(pytest stop after first failure)
pytest -x

(pytest run single test)
pytest -k ‘test_the_thing’

(pytest run single file)
pytest path/to/file.py

(ipython reload, autoreload, repl)
%load_ext autoreload
%autoreload 2
Or add this to ~/.ipython/profile_default/ipython_config.py to automatically
reload on every session:
c.InteractiveShellApp.exec_lines = [‘%load_ext autoreload’, ‘%autoreload 2’]

(ipython profile, python profile)
%prun some_function

(virtual environment, python2)
$ virtualenv .env

(python create environment, python virtual environment, python3)
python -m venv .env
(add env to gitignore)
source .env/bin/activate
see ~/notes/jupyter.txt

(venv, virtual env, requirements, freeze)
pip freeze > requirements.txt

(python env, git)
Add .env to .gitignoremy_environment
python -m venv .env
source .env/bin/activate
pip install -r requirements.txt


(ipython, matplotlib, plots, graph, math)
ipython –matplotlib
See ~/dev/snippets/python/matplotlib_experiment.py for an example

Within the plot window, use control to pan (drag along the x or y axis), or use
the zoom rect feature on the toolbar

(ipdb, single variable names)
Say you have a variable ‘n’. Use exclamation point in front:
ipdb> !n

(string match, methods matching, array select, list includes)
Find all methods that have pid in it:
[x for x in dir(os) if re.search(‘pid’, x, re.I)] # => re.I for ignore case

(getting help, help text, helptext, docstring)
> help(s.listen)
> print(s.listen.__doc__)

(print, lambda)
from future import print_function

Getting list of methods:
> dir(StringIO.StringIO)

Empty class implementation:
class Foo:


  Get class name:  
  ipdb> x.__class__  
  ipdb> x.__class__.__name__  


  $ pip install numpy  
  $ pip install matplotlib  
  $ pip install ipdb   # => debugger  


              $ pip install jedi  
~/.vim/bundle $ git clone --recursive https://github.com/davidhalter/jedi-vim.git  

Interactive python:

$ python -i  
$ ipython  

System python installs to
pip installs things to: /usr/local/lib/python2.7/site-packages

Pretty printing:

import pprint  
params = {  
    'latitude': 37.775818,  
    'longitude': -122.418028,  
    'server_token': 'snip'  
pp = pprint.PrettyPrinter(indent=4, width=1).pprint  

Pretty printing something that resembles a dictionary

pp(dict(response.headers))   # => note the width of 1 above is important  

(python debugger, ipdb, list, patch):
In /home/lou/dragon/lib/python2.7/site-packages/IPython/core/debugger.py,
Changed context from 3 to 10:

def print_stack_entry(self,frame_lineno,prompt_prefix='\n-> ',  
                      context = 10):  

Configuring iPython

Create default profile:  
$ ipython profile create  
$ ipython locate profile  

iPython tricks

Run a script:  
> %run workbench.py  

Insert an enter (newline) in repl:

ctrl+v + ctlr+j  

Also try %edit

Exec from command line

python -c "print('sup')"  


import sys  

Entry detection

if __name__ == '__main__':  
    print("yes, I am main")