python-mastery/Exercises/soln3_4.md

# Exercise 3.4 - Solution

## (a) Private attributes

```python
# stock.py

class Stock:
    _types = (str, int, float)
    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price

    @classmethod
    def from_row(cls, row):
        values = [func(val) for func, val in zip(cls._types, row)]
        return cls(*values)

    def cost(self):
        return self.shares * self.price

    def sell(self, nshares):
        self.shares -= nshares
```


## (b) Computed Attributes

```python
# stock.py

class Stock:
    _types = (str, int, float)
    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price

    @classmethod
    def from_row(cls, row):
        values = [func(val) for func, val in zip(cls._types, row)]
        return cls(*values)
    
    @property
    def cost(self):
        return self.shares * self.price

    def sell(self, nshares):
        self.shares -= nshares
```

## (c) Enforcing Type-Checking Rules

```python
# stock.py

class Stock:
    _types = (str, int, float)
    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price

    @classmethod
    def from_row(cls, row):
        values = [func(val) for func, val in zip(cls._types, row)]
        return cls(*values)
    
    @property
    def shares(self):
        return self._shares
    @shares.setter
    def shares(self, value):
        if not isinstance(value, int):
            raise TypeError('Expected an integer')
        if value < 0:
            raise ValueError('shares must be >= 0')
        self._shares = value

    @property
    def price(self):
        return self._price
    @price.setter
    def price(self, value):
        if not isinstance(value, float):
            raise TypeError('Expected a float')
        if value < 0:
            raise ValueError('price must be >= 0')
        self._price = value

    @property
    def cost(self):
        return self.shares * self.price

    def sell(self, nshares):
        self.shares -= nshares
```

## (d) Adding `__slots__`

```python
# stock.py

class Stock:
    __slots__ = ('name','_shares','_price')
    _types = (str, int, float)

    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price

    @classmethod
    def from_row(cls, row):
        values = [func(val) for func, val in zip(cls._types, row)]
        return cls(*values)

    @property
    def shares(self):
        return self._shares
    @shares.setter
    def shares(self, value):
        if not isinstance(value, int):
            raise TypeError('Expected integer')
        if value < 0:
            raise ValueError('shares must be >= 0')
        self._shares = value

    @property
    def price(self):
        return self._price
    @price.setter
    def price(self, value):
        if not isinstance(value, float):
            raise TypeError('Expected float')
        if value < 0:
            raise ValueError('price must be >= 0')
        self._price = value

    @property
    def cost(self):
        return self.shares * self.price

    def sell(self, nshares):
        self.shares -= nshares
```

## (e) Reconciling types

```python
# stock.py

class Stock:
    __slots__ = ('name','_shares','_price')
    _types = (str, int, float)
    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price

    @classmethod
    def from_row(cls, row):
        values = [func(val) for func, val in zip(cls._types, row)]
        return cls(*values)

    @property
    def shares(self):
        return self._shares
    @shares.setter
    def shares(self, value):
        if not isinstance(value, self._types[1]):
            raise TypeError(f'Expected {self._types[1].__name__}')
        if value < 0:
            raise ValueError('shares must be >= 0')
        self._shares = value

    @property
    def price(self):
        return self._price
    @price.setter
    def price(self, value):
        if not isinstance(value, self._types[2]):
            raise TypeError(f'Expected {self._types[2].__name__}')
        if value < 0:
            raise ValueError('price must be >= 0')
        self._price = value

    @property
    def cost(self):
        return self.shares * self.price

    def sell(self, nshares):
        self.shares -= nshares
```



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