CSE 163

More Objects�

Suh Young Choi�

🎶 Listening to: Shang-Chi soundtrack

💬 Before Class: Are animals better when they’re orbular or angular (aesthetically)?

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This Time

• Private fields and methods
• Default parameters

Last Time

• Objects (state and behavior)
• References
• Defining a class
• Instantiating an object

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Private

• Python has no way to actually enforce this, but by convention people don’t access things that start with “_”

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class Dog:

def __init__(self, name: str) -> None:

self._name: str = name

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def bark(self) -> None:

print(self._name + ': Woof')

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Private

• Both fields and methods can be private if you don’t want them accessed or changed by your user
• Private methods are often helper functions that your user doesn’t actually need

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class SpiderFolk:

def __init__(self, name: str) -> None:

self._name: str = name

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def _secret_identity(self) -> str:

if ‘spider’ not in self._name.lower():

return “Spider-Man!”

else:

return “who?”

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def get_me_pictures(self, count: int) -> None:

print(f’{count} pictures of {self._secret_identity()}’)

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“Getter” vs. “Setter” methods

• Two broad categories of class functions that involve field manipulation
• “Getter” methods return certain values, like fields or quick manipulations
• “Setter” methods update fields or create new values

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class SpiderFolk:

def __init__(self, name: str) -> None:

self._name: str = name

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def get_name(self) -> str:

return self._name # a getter method

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def rename(self, new_name: str) -> None:

self._name = new_name + “(renamed)” # a setter method

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Default Parameters

• You can use default parameters like you would before
• You have to be careful when using objects as default values, it has some really bad unintentional side-effects

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• There is only one instance of the default parameter
• In other words, they share a reference!

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def append_to(element: int,

to: list[int] = []) -> list[int]:

to.append(element)

return to

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my_list = append_to(12)

print(my_list)

my_other_list = append_to(42)

print(my_other_list)

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Default Parameters Done Right

• The fix is to not use an object as the default parameter, instead we usually use None

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# Option 1

def append_to(element: int,

to: list[int] | None = None) -> list[int]:

if to is None:

to = []

to.append(element)

return to

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# Option 2

def append_to(element, to=None):

to = [] if to is None else to

to.append(element)

return to

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Lambdas

• It’s kind of annoying to have to define a whole function just for this. It would be nice if there was a shorthand syntax

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Introduce: Lambda functions– these let us define functions within the body of our code!

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def get_dog_name(d):

return d.get_name()

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sorted(dogs, key=get_dog_name)

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sorted(dogs, key=lambda d: d.get_name())

Before Next Time

• HW3 + LR3 due tomorrow
• Go to section!
• New resubmission cycle open

Next Time

• Back in the classroom on Friday!
• Magic methods
• HW4 preview

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