Pythonic code

## Python Frederick April 2017

???

You, yeah, you reading the presenter notes. You can run this presentation by running:

$ python -m SimpleHTTPServer

from this directory in a terminal.

What is
Pythonic code?

??? Topics we'll cover

specific style guidance (list comprehension, with statement, property decorator)
general guidance (built-ins, standard library, pep8)

Key question to answer. But first...

Who are you?
What's a bit of your background?
Where are people at with their Python experience?
What are you hoping to learn?

???

Let's talk about lists.

[]

???

Python has a few core data types: tuple, list, dictionary

List crash course

empty = []
numbers = [1, 2, 3, 4, 5]
numbers.append(6)  # Add to a list
len(numbers)  # How many are in the list?
# Lists can mix types.
mixed = [1, 2, 'foo', 'bar']
three = numbers[2]

???

Describe the operations.

How would you created a doubled list of numbers?

Double a list of numbers
(C++)

int size = 5;
int numbers[size] = {1, 2, 3, 4, 5};
int doubled[size];
for (int i = 0; i < size; i++) {
    doubled[i] = 2 * numbers[i];
}

Double a list of numbers
(1st attempt)

numbers = [1, 2, 3, 4, 5]
doubled = []
size = len(numbers)
i = 0
while i < size:
    doubled.append(2 * numbers[i])
    i += 1

Double a list of numbers
(2nd attempt)

numbers = [1, 2, 3, 4, 5]
doubled = []
for number in numbers:
    doubled.append(2 * number)

???

This is a reasonable solution.

Double a list of numbers
(3rd attempt)

numbers = [1, 2, 3, 4, 5]
doubled = [2 * number for number in numbers]

???

7 lines down to 2. Theme of Pythonic code being short code too.

This is a list comprehension.

??? doubled is created with a list comprehension.

List comprehension anatomy

result_list = [
    expression for value in original_list]

List constructor - square brackets

An expression - lambda function that receives a value

An iterator over a list

Other cool list comprehensions

Filtering

numbers = [1, 2, 3, 4, 5]
odds = [number for number in numbers
               if number % 2 != 0]

Nested iteration

xs = [1, 2, 3, 4, 5]
ys = [1, 2, 3, 4, 5]
pairs = [(x, y) for x in xs
                for y in ys]

Open a file

???

Of course, we're going to look at... cat pics.

Open a file
(1st attempt)

f = open('kitteh.jpg', 'rb')
cat_pic = f.read()
# Do other stuff with the cat picture.

???

Missing the critical close
IOError "too many files open"

Open a file
(2nd attempt)

f = open('kitteh.jpg', 'rb')
cat_pic = f.read()
# Do other stuff with the cat picture.
f.close()

Open a file
(3rd attempt)

try:
    f = open('kitteh.jpg', 'rb')
    cat_pic = f.read()
    # Do other stuff with the cat picture.
    f.close()
except:
    print('oops, something went wrong.')

???

Who can spot the bug?
Yuck, please don't catch all exceptions like this.

Open a file
(4th attempt)

try:
    f = open('kitteh.jpg', 'rb')
    cat_pic = f.read()
    # Do other stuff with the cat picture.
except:
    print('oops, something went wrong.')
finally:
    f.close()

Open a file
(5th attempt)

with open('kitteh.jpg', 'rb') as f:
    cat_pic = f.read()
try:
    # Do other stuff with the cat picture.
except:
    print('oops, something went wrong.')

???

Notice that something is absent, where is close?

with

The with statement

When using with, open automatically closes files. --
Work is passed to a "context manager." --
A context manager defines __enter__ and __exit__. --
Examples: files, locks, timers --
Write your own with contextlib.

???

Do your classes look like this?

Clean up a class

class OhMyJava:
    def __init__(self):
        self._foo = ''
    def get_foo(self):
        return self._foo
    def set_foo(self, foo):
        self._foo = foo

oh_my = OhMyJava()
oh_my.set_foo('This is painful.')

Clean up a class (1st attempt)

class AwwYeah:
    def __init__(self):
        self.foo = ''

If you aren't doing anything special, it is safe to set the attribute directly.

But, encapsulation!

???

Setting attributes directly seems to violate encapsulation.
Encapsulation is supposed to be a core benefit of object oriented design.

Clean up a class (2nd attempt)

class AwwYeah:
    def __init__(self):
        self._bar = ''
    @property
    def foo(self):
        return 'Wow: {}'.format(self._bar)
    @foo.setter
    def foo(self, value):
        self._bar = '{}!'.format(value)

>>> a = AwwYeah()
>>> a.foo = 'Python'
>>> a.foo
'Wow: Python!'

The property decorator

???

Fewer parenthesis means less to comprehend.
Excluding a setter makes ready only properties. Handy.
Properties are super useful for computed/derived values. --
@property transforms obj.foo() to obj.foo --
Use of property make another decorator available: @foo.setter --
Computed/derived values.

Use the core

Google search: python built in functions --
Always globally available functions --
Some favorites:
- any
- enumerate
- print
- range
- sorted

any

>>> checks = [False, False, True, False]
>>> status = any(checks)
>>> status
True

enumerate

# Bad variable name. Short for slides.
>>> f = ['apple', 'orange', 'kiwi', 'pear']
>>> for idx, fruit in enumerate(f, start=1):
...     print(idx, fruit)
...
(1, 'apple')
(2, 'orange')
(3, 'kiwi')
(4, 'pear')

???

Default start is 0.

print

>>> print('Hello World')
Hello World
>>> import sys
>>> print('An error!', file=sys.stderr)
An error!

???

This is one of the big difference between Python 2 and 3.
In Python 2, print was a keyword.
Now, it's much easier to use keyword arguments and pass to different streams.

range

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(5, 10)
[5, 6, 7, 8, 9]
>>> range(0, 10, 2)
[0, 2, 4, 6, 8]

???

Step is very infrequently used.

sorted

>>> numbers = [5, 2, 3, 1, 4]
>>> ordered = sorted(numbers)
>>> ordered
[1, 2, 3, 4, 5]
>>> numbers
[5, 2, 3, 1, 4]
>>> numbers.sort()
>>> numbers
[1, 2, 3, 4, 5]

???

Sorted in new list.
In place sorting

Batteries included

???

The language wants to enable you by including the things you'll probably need anyway. --
Python has a huge standard library. --
There are tools for:
- Manipulating files (os.path) --
- Matching patterns (re) --
- Handling dates and times (datetime) --
- Creating CLIs (argparse) --
- Working with CSV (csv) --
- and tons more!

What did we cover?

list comprehensions
the with statement
the property decorator
built-ins
the standard library

What is
Pythonic code?

Pythonic code is code that takes advantage of the features of the Python language and feels more natural than implementing language X in Python.

Resources

Thank you

Matt Layman @mblayman

mattlayman.com

GeezerVison

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README.md

Pythonic code

What is Pythonic code?

[]

List crash course

Double a list of numbers (C++)

Double a list of numbers (1st attempt)

Double a list of numbers (2nd attempt)

Double a list of numbers (3rd attempt)

This is a list comprehension.

List comprehension anatomy

Other cool list comprehensions

Filtering

Nested iteration

Open a file

Open a file (1st attempt)

Open a file (2nd attempt)

Open a file (3rd attempt)

Open a file (4th attempt)

Open a file (5th attempt)

with

The with statement

Clean up a class

Clean up a class (1st attempt)

But, encapsulation!

Clean up a class (2nd attempt)

The property decorator

Use the core

any

enumerate

print

range

sorted

Batteries included

What did we cover?

What is Pythonic code?

Resources

Thank you

What is
Pythonic code?

Double a list of numbers
(C++)

Double a list of numbers
(1st attempt)

Double a list of numbers
(2nd attempt)

Double a list of numbers
(3rd attempt)

Open a file
(1st attempt)

Open a file
(2nd attempt)

Open a file
(3rd attempt)

Open a file
(4th attempt)

Open a file
(5th attempt)

What is
Pythonic code?