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David Tan and Emma Sun Update lesson plan 67a9af3 Jul 19, 2019
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Functions

Use functions to keep code "DRY"

The developer who learns to recognize duplication, and understands how to eliminate it through proper abstraction (i.e. defining the right functions or methods), can produce much cleaner code than one who continuously infects the application with unnecessary repetition.

Every line of code that goes into an application must be maintained, and is a potential source of future bugs. Duplication needlessly bloats the codebase, resulting in more opportunities for bugs and adding accidental complexity to the system. The bloat that duplication adds to the system also makes it more difficult for developers working with the system to fully understand the entire system, or to be certain that changes made in one location do not also need to be made in other places that duplicate the logic they are working on. DRY requires that "every piece of knowledge must have a single, unambiguous, authoritative representation within a system."

Source: 97 Things Every Programmer Should Know

Bad:

decision_tree_model = DecisionTreeClassifier()
decision_tree_model.fit(X_train, Y_train)
Y_pred = decision_tree_model.predict(X_test)
decision_tree_accuracy = round(decision_tree_model.score(X_train, Y_train) * 100, 2)
print(decision_tree_accuracy)

random_forest_model = RandomForestClassifier(n_estimators=100)
random_forest_model.fit(X_train, Y_train)
Y_pred = random_forest_model.predict(X_test)
random_forest_model.score(X_train, Y_train)
random_forest_accuracy = round(random_forest_model.score(X_train, Y_train) * 100, 2)
print(random_forest_accuracy)

gaussian_model = GaussianNB()
gaussian_model.fit(X_train, Y_train)
Y_pred = gaussian_model.predict(X_test)
gaussian_accuracy = round(gaussian_model.score(X_train, Y_train) * 100, 2)
print(gaussian_accuracy)

Good:

def train_model(ModelClass, X_train, Y_train, **kwargs):
    model = ModelClass(**kwargs)
    model.fit(X_train, Y_train)
    
    accuracy_score = round(model.score(X_train, Y_train) * 100, 2)
    print(f'accuracy ({ModelClass.__name__}): {accuracy_score}')
    
    return model, accuracy_score

decision_tree_model, decision_tree_accuracy = train_model(DecisionTreeClassifier, X_train, Y_train)
random_forest_model, random_forest_accuracy = train_model(RandomForestClassifier, X_train, Y_train, n_estimators=100)
gaussian_model     , gaussian_accuracy      = train_model(GaussianNB, X_train, Y_train)

Tip: Notice how the symmetry of the 3 code blocks in the bad example made it easier for us to identify and refactor the duplicated code? One useful practice in eliminating duplication is to first make the duplication as obvious as possible. This makes it easier for us to identify opportunities for extracting the duplication into their appropriate homes.

Functions should do one thing

This is by far the most important rule in software engineering. When functions do more than one thing, they are harder to compose, test, and reason about. When you can isolate a function to just one action, they can be refactored easily and your code will read much cleaner. If you take nothing else away from this guide other than this, you'll be ahead of many developers.

Bad:

def email_clients(clients: List[Client]):
    """Filter active clients and send them an email.
    """
    for client in clients:
        if client.active:
            email(client)

Good:

def get_active_clients(clients: List[Client]) -> List[Client]:
    """Filter active clients.
    """
    return [client for client in clients if client.active]


def email_clients(clients: List[Client, ...]) -> None:
    """Send an email to a given list of clients.
    """
    for client in clients:
        email(client)

Do you see an opportunity for using generators now?

Even better

def active_clients(clients: List[Client]) -> Generator[Client]:
    """Only active clients.
    """
    return (client for client in clients if client.active)


def email_client(clients: Iterator[Client]) -> None:
    """Send an email to a given list of clients.
    """
    for client in clients:
        email(client)

Functions should only be one level of abstraction

When you have more than one level of abstraction, your function is usually doing too much. Splitting up functions leads to reusability and easier testing.

Bad:

def parse_better_js_alternative(code: str) -> None:
    regexes = [
        # ...
    ]

    statements = regexes.split()
    tokens = []
    for regex in regexes:
        for statement in statements:
            # ...

    ast = []
    for token in tokens:
        # Lex.

    for node in ast:
        # Parse.

Good:

def parse_better_js_alternative(code: str) -> None:
    tokens = tokenize(code)
    syntax_tree = parse(tokens)

    for node in syntax_tree:
        # Parse.


def tokenize(code: str) -> list:
    REGEXES = [
        # ...
    ]

    statements = code.split()
    tokens = []
    for regex in REGEXES:
        for statement in statements:
           # Append the statement to tokens.

    return tokens


def parse(tokens: list) -> list:
    syntax_tree = []
    for token in tokens:
        # Append the parsed token to the syntax tree.

    return syntax_tree

Function names should say what they do

Bad:

class Email:
    def handle(self) -> None:
        # Do something...

message = Email()
# What is this supposed to do again?
message.handle()

Good:

class Email:
    def send(self) -> None:
        """Send this message.
        """

message = Email()
message.send()

Use type hints to improve readability

Using type hints can make your code more readable and reasonable. Your development experience will also be improved because your IDE will be able to give you better auto-complete suggestions about function/method names and parameters.

Bad:

Without type hints, we are forced to embed type information in variable names (e.g. pd_series). This can make variable names unnecessarily long. Also, our IDE is not able to give us auto-complete hints, and as a result, we have to hop to the source file to find out what parameters this function accepts.

type hints bad example

Good:

With type hints, we can name our variables sensibly, and the IDE now offers better autocompletion and make us more productive and less error-prone.

type hints good example

It's important to note that type hints are meant to be entirely ignored by the Python runtime, and are checked only by 3rd party tools like mypy and Pycharm's integrated checker. You can read more about type hints and how to use type checkers here.

Avoid side effects

A function produces a side effect if it does anything other than take a value in and return another value or values. For example, a side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a stranger.

Now, you do need to have side effects in a program on occasion - for example, like in the previous example, you might need to write to a file. In these cases, you should centralize and indicate where you are incorporating side effects. Don't have several functions and classes that write to a particular file - rather, have one (and only one) service that does it.

The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, or using an instance of a class, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.

Bad:

# Global variable referenced by following function.
# If another function used this name, now it'd be an array and could break.
name = 'Ryan McDermott'

def split_into_first_and_last_name() -> None:
    global name
    name = name.split()

split_into_first_and_last_name()
print(name)  # ['Ryan', 'McDermott']

# calling this function the second time will throw AttributeError: 'list' object has no attribute 'split'
split_into_first_and_last_name()

Good:

def split_into_first_and_last_name(name: str) -> None:
    return name.split()

name = 'Ryan McDermott'
new_name = split_into_first_and_last_name(name)

print(name)  # 'Ryan McDermott'
print(new_name)  # ['Ryan', 'McDermott']

Avoid unexpected side effects on values passed as function parameters

We can unexpectedly change the values passed to our functions, even though our functions appear to be pure.

This will happen when we pass non-primitive objects (e.g. lists, dictionaries, instances of classes, pandas dataframes) to a function because in Python (and indeed many other languages), non-primitive objects are passed by reference.

Bad:

import pandas as pd

original = pd.DataFrame({
    'values': [1,2,3],
})

def multiply_column_by_10(df, column_name):
    df['multiplied column'] = df[column_name] * 10
    
    return df
    
new = multiply_column_by_10(original, 'values')

original.head() # surprise! original dataframe is mutated and now it has

Good:

import pandas as pd

original = pd.DataFrame({
    'values': [1,2,3],
})

def multiply_column_by_10(df, column_name):
    df = df.copy()
    df['multiplied column'] = df[column_name] * 10
    
    return df
    
new = multiply_column_by_10(original, 'values')

original.head() # original dataframe is not mutated

Function arguments (2 or fewer ideally)

Limiting the amount of function parameters is incredibly important because it makes testing your function easier. Having more than three leads to a combinatorial explosion where you have to test tons of different cases with each separate argument.

One or two arguments is ok, and three should be avoided. Anything more than that should be consolidated. Usually, if you have more than two arguments then your function is trying to do too much. In cases where it's not, most of the time a higher-level object will suffice as an argument.

Bad:

def create_menu(title, body, button_text, cancellable):
    # ...

Good:

class Menu:
    def __init__(self, config: dict):
        title = config["title"]
        body = config["body"]
        # ...

menu = Menu(
    {
        "title": "My Menu",
        "body": "Something about my menu",
        "button_text": "OK",
        "cancellable": False
    }
)

Also good

class MenuConfig:
    """A configuration for the Menu.

    Attributes:
        title: The title of the Menu.
        body: The body of the Menu.
        button_text: The text for the button label.
        cancellable: Can it be cancelled?
    """
    title: str
    body: str
    button_text: str
    cancellable: bool = False


def create_menu(config: MenuConfig):
    title = config.title
    body = config.body
    # ...


config = MenuConfig
config.title = "My delicious menu"
config.body = "A description of the various items on the menu"
config.button_text = "Order now!"
# The instance attribute overrides the default class attribute.
config.cancellable = True

create_menu(config)

Fancy

from typing import NamedTuple


class MenuConfig(NamedTuple):
    """A configuration for the Menu.

    Attributes:
        title: The title of the Menu.
        body: The body of the Menu.
        button_text: The text for the button label.
        cancellable: Can it be cancelled?
    """
    title: str
    body: str
    button_text: str
    cancellable: bool = False


def create_menu(config: MenuConfig):
    title, body, button_text, cancellable = config
    # ...


create_menu(
    MenuConfig(
        title="My delicious menu",
        body="A description of the various items on the menu",
        button_text="Order now!"
    )
)

Even fancier

from dataclasses import astuple, dataclass


@dataclass
class MenuConfig:
    """A configuration for the Menu.

    Attributes:
        title: The title of the Menu.
        body: The body of the Menu.
        button_text: The text for the button label.
        cancellable: Can it be cancelled?
    """
    title: str
    body: str
    button_text: str
    cancellable: bool = False

def create_menu(config: MenuConfig):
    title, body, button_text, cancellable = astuple(config)
    # ...


create_menu(
    MenuConfig(
        title="My delicious menu",
        body="A description of the various items on the menu",
        button_text="Order now!"
    )
)

Use default arguments instead of short circuiting or conditionals

Tricky

Why write:

def create_micro_brewery(name):
    name = "Hipster Brew Co." if name is None else name
    slug = hashlib.sha1(name.encode()).hexdigest()
    # etc.

... when you can specify a default argument instead? This also makes ist clear that you are expecting a string as the argument.

Good:

def create_micro_brewery(name: str = "Hipster Brew Co."):
    slug = hashlib.sha1(name.encode()).hexdigest()
    # etc.

Don't use flags as function parameters

Flags tell your user that this function does more than one thing. Functions should do one thing. Split your functions if they are following different code paths based on a boolean.

Bad:

from pathlib import Path

def create_file(name: str, temp: bool) -> None:
    if temp:
        Path('./temp/' + name).touch()
    else:
        Path(name).touch()

Good:

from pathlib import Path

def create_file(name: str) -> None:
    Path(name).touch()

def create_temp_file(name: str) -> None:
    Path('./temp/' + name).touch()
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