Learn Python, the Right Way (Workshop)

Let’s Build!

Demo: accountant.py This is what we plan to build.

$ python3 -‐m venv accountant_venv $ . accountant_venv/bin/activate
(accountant_venv) $ > python3 -‐m venv accountant_venv > accountant_venv\Scripts\activate (accountant_venv) >

project !"" accountant_venv # !"" bin
# !"" include # !"" lib # $"" pyvenv.cfg $"" accountant.py

Function to add an item

def _show_items(items): template = '{name:20}{price:>5}'
if not items: print("No items. Use 'add' to add some!") return print() print(template.format(name='Item', price='Price')) print('-‐' * 25) for item in items: print(template.format( name=item['name'], price=item['price'], )) print()

def add(): items = []
name = input('Item name: ') price = input('Price: ') if name and price: items.append({ 'name': name, 'price': price, }) _show_items(items) if __name__ == '__main__': add()

http://d.pr/1c5Bv

$ python accountant.py What to buy: Apples Price:
10 Item Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10

Persistence

File Objects • For reading ﬁles and ﬁle-like interfaces •
Built-in type file • close(), read(), readline(), seek(), etc.

Path Objects • import pathlib • pathlib.Path • Path.cwd() •
desktop = home / 'Desktop' • home = desktop.parent

import pathlib path = pathlib.Path('input.txt') f = path.open()
line = f.readline() while line: print(line) line = f.readline() f.close() print('ALL DONE')

import pathlib path = pathlib.Path('input.txt') f = path.open()
for line in f: print(line) f.close() print('ALL DONE')

import pathlib pathlib.Path('input.txt') with path.open() as f:
for line in f: print(line) print('ALL DONE') File closed automatically on exiting.

import pathlib path = pathlib.Path('output.txt') with path.open('w') as
f: f.write('Hello world!\n') print('ALL DONE')

The json module • import json • json.dump(obj, f) •
json.load(f) 㶷Ⰵ 隟⳿

Python types JSON types int, float Number str String list,
tuple, etc. Array dict, etc. Object None null

import pathlib def _get_save_path(): my_path
= pathlib.Path(__file__) save_path = my_path.parent / 'items.json' return save_path

import json def _load_items(): path
= _get_save_path() try: with path.open() as f: items = json.load(f) except (FileNotFoundError, ValueError): items = [] return items

def _save_items(items): # Your turn!! Implement this.
def add(): items = _load_items() # Original implementation. _save_items(items)

project !"" accountant_venv # !"" ...
!"" accountant.py $"" items.json

$ python accountant.py What to buy: Apples Price:
10 Item Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 $ python accountant.py What to buy: Oranges Price: 15 Item Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 Oranges 15

Command Line Interface

$ python accountant.py add What to buy: Bananas
Price: 7 Item Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 Oranges 15 Bananas 7 $ python accountant.py show Item Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 Oranges 15 Bananas 7

$ pip install click Collecting click Downloading
click-‐4.0-‐py2.py3-‐none-‐any.whl (62kB) 100% |████████████████████████| 65kB 1.1MB/s Installing collected packages: click Successfully installed click-‐4.0 $

pip • Python’s package manager • Manages packages on PyPI
(Python Package Index) • pip install, uninstall, freeze

https://pypi.python.org/

http://click.pocoo.org/

$ pip freeze > requirements.txt

project !"" accountant_venv # !"" ...
!"" accountant.py !"" items.json $"" requirements.txt

Decorators • Modify a callable without re-deﬁning • “Wraps” the
target • How is that even possible!?

def hi(name): print('Hi' + name +'!')
greet = hi greet('Mosky') First-class Functions

def get_name(): return 'TP' def
hi(name_getter, words): def make_message(words): return ' '.join(words) print('{msg}, {name}!'.format( msg=make_message(words), name=name_getter(), )) hi(get_name, ['Hello', 'there'])

def hi(name, words): def make_message():
return ' '.join(words) print('{msg}, {name}!'.format( msg=make_message(words), name=name, )) # Prints "Hello there, people!" hi('people', ['Hello', 'there'])

def outer_wrapper(func) def inner_wrapper(name):
print('Before') func(name) print('After') return inner_wrapper def hi(name): print('Hi, {}!'.format(name)) # What happens here? outer_wrapper(hi)('Marty')

print('Before') func(name) print('After') return inner_wrapper def hi(name): print('Hi, {}!'.format(name)) hi = outer_wrapper(hi) hi('Marty')

print('Before') func(name) print('After') return inner_wrapper @outer_wrapper def hi(name): print('Hi, {}!'.format(name)) hi('Marty')

import click @click.group() def cli():
pass @cli.add_command @click.command() def add(): # Implementation does not change. if __name__ == '__main__': cli()

$ python accountant.py add What to buy: Bananas
Price: 7 Item Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 Oranges 15 Bananas 7

$ python accountant.py # What happens here? $
python accountant.py -‐-‐help # What happens here?

$ python accountant.py show Item
Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 Oranges 15 Bananas 7

Item Removal

$ python accountant.py show Item
Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 Oranges 15 Bananas 7 $ python accountant.py remove 1 Item Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ Apples 10 Bananas 7

# remove_test.py def remove(index): items
= [ {'name': 'Apples', 'price': 10}, {'name': 'Oranges', 'price': 15}, ] # 㻛⡲... # 㹀纏湫䱸䖰 accountant.py 䭠頺麓⢵կ _show_items(items) if __name__ = '__main__': index = input('Index: ') remove(index)

Keyword Arguments • print('Hello', 'World', sep=', ') • Named and
unnamed arguments • Most arguments can be either

def hi(name, message): print('{}, {}!'.format(message, name))
# Calling with positional arguments. hi('Doc', 'Hello') # Calling with keyword arguments. hi(message='Hello', name='Doc')

import sys print('Hello', 'World',
sep=', ', file=sys.stderr) Variable-length positional arguments keyword-only arguments

https://docs.python.org/3/ tutorial/controlﬂow.html#more- on-deﬁning-functions

@cli.add_command @click.command() @click.argument('index') def remove(index):
items = _load_items() # ... # Before _show_items(items) _save_items(items)

$ python accountant.py # What happens now? $
python accountant.py remove 0 # What happens? $ python accountant.py remove # What happens?

Reﬁning UI

$ python accountant.py show # Item
Price -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 0 Apples 10 1 Oranges 15 2 Bananas 7

def _show_items(items): template = '{i:>3}
{name:20}{price:>5}' # ... for i in range(len(items)): item = items[i] print(template.format( i=i, name=item['name'], price=item['price'], )) print()

“There must be a better way.”

{name:20}{price:>5}' # ... i = 0 for item in items: print(template.format( i=i, name=item['name'], price=item['price'], )) i += 1 print()

{name:20}{price:>5}' # ... for i, item in zip(range(len(items)), items): print(template.format( i=i, name=item['name'], price=item['price'], )) print()

{name:20}{price:>5}' # ... for i, item in enumerate(items): print(template.format( i=i, name=item['name'], price=item['price'], )) print()

Next Steps • Conﬁgurable _get_save_path • configparser module • Save
as SQLite and other formats • sqlite3 module • Even more CLI improvements • colorama and palpatine • Any other ideas? Ask!

Learn Python, the Right Way (Workshop)

Learn Python, the Right Way (Workshop)

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