pythonic编写此代码的方式

Question

I have this ugly piece of code..

 orders = got_something_from_db()
 if orders:
    for order in orders:
        #print order
        event_id = order["event_id"]
        if event_id in event_id_dict: # something i grabbed earlier
            product_id = order["product_id"] # products in an event
            qty = order["qty"]
            if product_id in product_sku_dict: 
                sku_id =product_sku_dict[product_id]
                for i in range(qty):
                    sku_ids.append(sku_id)

How do I made this more pythonic (and terse)

Answer 1

First, the code is not exactly bad - it is clear enough to read and understand and so you have definitely passed more than the first hurdle of maintainability.

If there is a problem it is the deep nesting. I would break this up by making a number of functions, each with a distinct business purpose expressed in their name:

def order_has_valid_event(order):
    """Returns True if the order was raised by an event
    in event_id_dict"""
    event_id = order["event_id"]
    return event_id in event_id_dict # something i grabbed earlier

def get_sku_from_order(order):
    """Return the SKU of the product in an order"""
    product_id = order['product_id']
    try:
        return product_sku_dict[product_id]
    except KeyError:
        raise KeyError("Product {0} is not in SKU dictionary".format(product_id))

def get_order_skus(order):
    """Returns a list of SKUs in an order"""
    sku_id = get_sku_from_order(order)
    qty = order["qty"]
    return [sku_id] * qty

# Modify got_something_from_db to always return a list, even if empty 
orders = got_something_from_db()
for order in orders:
    #print order
    if order_has_valid_event(order):
        try:
            sku_ids.extend(get_order_skus(order))
        except KeyError:
            continue

Please forgive this attempt if the names don't quite match your intent - just rename them appropriately.

My attempted improvements:

• Instead of looping qty times appending, you can create a list of qty elements with the list multiply: ['a'] * 3 = ['a', 'a', 'a']
• At a business level, I would question the silently ignored error when the product id is not in the SKU dictionary. It is highly likely that this is a real issue (all products should have an SKU in the industries I have worked in). If so, let the error propagate up quickly and 'loudly' to either an appropriate error handler, or more likely a place where the order can be rejected. To support this more clearly, I have caught the plain KeyError in get_sku_from_order and thrown a more explicit exception message.
• Modify got_something_from_db to always return a list, even if empty. This simplifies the logic and flow

Answer 2

With limited context, here's the best I can do.

orders = got_something_from_db()
for order in orders: #make got something return empty iterable on failure
    if order["event_id"] in event_id_dict:
        product_id = order["product_id"]
        try:
            sku_id = product_sku_dict[product_id]
            #Change sku_ids to a collections.Counter (assuming order is unimportant)
            sku_ids[sku_id] += order["qty"] 
        except KeyError:
            pass

Also, consider changing event_id, product_id, etc to attributes. You probably want a namedtuple here, not a dict.

Answer 3

I'd structure it something like the following:

def add_product_to_skus(product_id, qty):
    if product_id in product_sku_dict: 
        sku_id = product_sku_dict[product_id]
        sku_ids.extend(qty*[sku_id])    

# ...

orders = got_something_from_db()
if not orders:
    return

valid_orders = (o for o in orders if o['event_id'] in event_id_dict)

for o in valid_orders:
    add_product_to_skus(o['product_id'], o['qty'])

Some golfing of the above would give you:

orders = got_something_from_db()
if not orders:
    return
add_products_to_skus((o['product_id'], o['qty']) for o in orders 
                     if o['event_id'] in event_id_dict 
                     if o['product_id'] in product_sku_dict)
# ...  
def add_product_to_skus(product_qtys):
    for product_id, qty in product_qtys:
        sku_id = product_sku_dict[product_id]
        sku_ids.extend(qty*[sku_id])

But the transformation from your original to this form isn't necessarily clear (or even correct), and the list comprehension would probably deserve a comment explaining the filtering.

Answer 4

sku_ids = []
event_ids = get_event_id_dict()
orders = got_something_from_db()

for order in orders:
    if order["event_id"] in event_ids:
        try:
            sku_id = product_sku_dict[order["product_id"]]
        except KeyError:
            continue

        sku_ids.extend([sku_id] * order["qty"])

Answer 5

You'll probably want to raise an exception rather than silently ignore a missing SKU item. This is how to be the most modular and elegant, not necessary pythonic:

database = ...
productId_to_sku = {...}

def productIdToSku(product_id):
    try:
        return productId_to_sku[product_id]
    except IndexError:
        raise Exception('product id #{} not in SKU database; have a manager add it'.format(product_id))

def getSkusFromEvent(event_id):
    orders = database.fetch(event_id=event_id)
    for order in orders:
        yield (productIdToSku(order.PRODUCT_ID), order.QTY)

collections.Counter(getSkusFromEvent(YOUR_EVENT_ID))