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自动选题[tech]: 20200914 Use Python to solve a charity's business problem
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[#]: collector: (lujun9972)
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[#]: translator: ( )
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Use Python to solve a charity's business problem)
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[#]: via: (https://opensource.com/article/20/9/solve-problem-python)
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[#]: author: (Chris Hermansen https://opensource.com/users/clhermansen)
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Use Python to solve a charity's business problem
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======
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Comparing how different programming languages solve the same problem is
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fun and instructive. Next up, Python.
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![Python programming language logo with question marks][1]
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In my [first article][2] in this series, I described a problem of dividing bulk supplies into hampers of similar value to distribute to struggling neighbors in your community. I also wrote about how I enjoy solving small problems like this with small programs in various languages and comparing how they do it.
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In the first article, I solved this problem with the [Groovy][3] programming language. Groovy is like [Python][4] in many ways, but syntactically it's more like C and Java. Therefore, it should be interesting and instructive to create the same solution in Python.
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### The Python solution
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In Java, I declare utility classes to hold tuples of data (the new record feature is going to be great for that). In Groovy, I use the language support for maps, and I follow the same approach in Python.
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Use a list of dictionaries to hold the bulk items picked up from the wholesaler:
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```
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packs = [
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{'item':'Rice','brand':'Best Family','units':10,'price':5650,'quantity':1},
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{'item':'Spaghetti','brand':'Best Family','units':1,'price':327,'quantity':10},
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{'item':'Sardines','brand':'Fresh Caught','units':3,'price':2727,'quantity':3},
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{'item':'Chickpeas','brand':'Southern Style','units':2,'price':2600,'quantity':5},
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{'item':'Lentils','brand':'Southern Style','units':2,'price':2378,'quantity':5},
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{'item':'Vegetable oil','brand':'Crafco','units':12,'price':10020,'quantity':1},
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{'item':'UHT milk','brand':'Atlantic','units':6,'price':4560,'quantity':2},
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{'item':'Flour','brand':'Neighbor Mills','units':10,'price':5200,'quantity':1},
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{'item':'Tomato sauce','brand':'Best Family','units':1,'price':190,'quantity':10},
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{'item':'Sugar','brand':'Good Price','units':1,'price':565,'quantity':10},
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{'item':'Tea','brand':'Superior','units':5,'price':2720,'quantity':2},
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{'item':'Coffee','brand':'Colombia Select','units':2,'price':4180,'quantity':5},
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{'item':'Tofu','brand':'Gourmet Choice','units':1,'price':1580,'quantity':10},
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{'item':'Bleach','brand':'Blanchite','units':5,'price':3550,'quantity':2},
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{'item':'Soap','brand':'Sunny Day','units':6,'price':1794,'quantity':2}]
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```
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There is one bulk pack of 10 bags of rice and 10 bulk packs with one bag each of spaghetti. In the above, the variable `packs` is set to a Python list of dictionaries. This turns out to be very similar to the Groovy approach. A few points worth noting about the difference between Groovy and Python:
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1. In Python, there is no keyword used to define the variable `packs`; Python expects the first use to set a value.
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2. Python dictionary keys (e.g., `item`, `brand`, `units`, `price`, `quantity`) require quotes to indicate they are strings; Groovy assumes these are strings, but accepts quotes as well.
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3. In Python, the notation `{ … }` indicates a dictionary declaration; Groovy uses the same square brackets as a list, but the structure in both cases must have key-value pairs.
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And, yes, those prices aren't in US dollars.
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Next, unpack the bulk packages. Unpacking the single bulk package of rice, for example, will yield 10 units of rice; that is, the total number of units yielded is `units * quantity`. The Groovy script uses a handy function called `collectMany` that can be used to flatten out lists of lists. As far as I know, Python doesn't have anything similar, so use two list comprehensions to produce the same result:
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```
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units = [[{'item':pack['item'],'brand':pack['brand'],
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'price':(pack['price'] / pack['units'])}] *
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(pack['units'] * pack['quantity']) for pack in packs]
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units = [x for sublist in units for x in sublist]
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```
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The first list comprehension (assignment to units) builds the list of lists of dictionaries. The second "flattens" that into just a list of dictionaries. Note that both Python and Groovy provide an `*` operator that takes a list on the left and a number `N` on the right and replicates the list `N` times.
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The final step is to repack the units into the hampers for distribution. As in the Groovy version, you need to get a bit more specific about the ideal hamper value, and you might as well not be overly restrictive when you get down to just a few units left:
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```
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valueIdeal = 5000
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valueMax = valueIdeal * 1.1
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```
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OK! Repack the hampers:
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```
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import random
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hamperNumber = 0 # [1]
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while len(units) > 0: # [2]
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hamperNumber += 1
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hamper = []
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value = 0
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canAdd = True # [2.1]
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while canAdd: # [2.2]
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u = random.randint(0,len(units)-1) # [2.2.1]
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canAdd = False # [2.2.2]
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o = 0 # [2.2.3]
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while o < len(units): # [2.2.4]
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uo = (u + o) % len(units)
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unit = units[uo]
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unitPrice = unit['price'] # [2.2.4.1]
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if len(units) < 3 or not (unit in hamper) and (value + unitPrice) < valueMax:
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# [2.2.4.2]
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hamper.append(unit)
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value += unitPrice
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units.pop(u) # [2.2.4.3]
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canAdd = len(units) > 0
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break # [2.2.4.4]
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o += 1 # [2.2.4.5]
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# [2.2.5]
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print('')
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print('Hamper',hamperNumber,'value',value)
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for item in hamper:
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print('%-25s%-25s%7.2f' % (item['item'],item['brand'],item['price'])) # [2.3]
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print('Remaining units',len(units)) # [2.4]
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```
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Some clarification, with numbers in brackets in the comments above (e.g., _[1]_) corresponding to the clarifications below:
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* 1\. Import Python's random number generator facilities and initialize the hamper number.
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* 2\. This `while` loop will redistribute units into hampers as long as there are more available:
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* 2.1 Increment the hamper number, get a new empty hamper (a list of units), and set its value to 0; start off assuming you can add more items to the hamper.
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* 2.2 This `while` loop will add as many units to the hamper as possible (the Groovy code used a `for` loop, but Python's `for` loops expect to iterate over something, while Groovy has the more traditional C form of `for` loop):
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* 2.2.1 Get a random number between zero and the number of remaining units minus 1.
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* 2.2.2 Assume you can't find more units to add.
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* 2.2.3 Create a variable to be used for the offset from the starting point where you're looking for items to put in the hamper.
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* 2.2.4 Starting at the randomly chosen index, this `while` loop will try to find a unit that can be added to the hamper (once again, note that the Python `for` loop probably isn't suitable here since the length of the list will change during processing).
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* 2.2.4.1. Figure out which unit to look at (random starting point + offset) and get its price.
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* 2.2.4.2 You can add this unit to the hamper if there are only a few left or if the value of the hamper isn't too high once the unit is added.
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* 2.2.4.3 Add the unit to the hamper, increment the hamper value by the unit price, remove the unit from the available units list.
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* 2.2.4.4 As long as there are units left, you can add more, so break out of this loop to keep looking.
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* 2.2.4.5 Increment the offset.
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* 2.2.5 On exit from this `while` loop, if you inspected every remaining unit and could not find one to add to the hamper, the hamper is complete; otherwise, you found one and can continue looking for more.
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* 2.3 Print out the contents of the hamper.
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* 2.4 Print out the remaining units info.
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When you run this code, the output looks quite similar to the output from the Groovy program:
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```
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Hamper 1 value 5304.0
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UHT milk Atlantic 760.00
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Tomato sauce Best Family 190.00
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Rice Best Family 565.00
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Coffee Colombia Select 2090.00
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Sugar Good Price 565.00
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Vegetable oil Crafco 835.00
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Soap Sunny Day 299.00
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Remaining units 148
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Hamper 2 value 5428.0
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Tea Superior 544.00
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Lentils Southern Style 1189.00
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Flour Neighbor Mills 520.00
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Tofu Gourmet Choice 1580.00
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Vegetable oil Crafco 835.00
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UHT milk Atlantic 760.00
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Remaining units 142
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Hamper 3 value 5424.0
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Soap Sunny Day 299.00
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Chickpeas Southern Style 1300.00
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Sardines Fresh Caught 909.00
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Rice Best Family 565.00
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Vegetable oil Crafco 835.00
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Spaghetti Best Family 327.00
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Lentils Southern Style 1189.00
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Remaining units 135
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…
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Hamper 21 value 5145.0
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Tomato sauce Best Family 190.00
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Tea Superior 544.00
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Chickpeas Southern Style 1300.00
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Spaghetti Best Family 327.00
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UHT milk Atlantic 760.00
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Vegetable oil Crafco 835.00
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Lentils Southern Style 1189.00
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Remaining units 4
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Hamper 22 value 2874.0
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Sardines Fresh Caught 909.00
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Vegetable oil Crafco 835.00
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Rice Best Family 565.00
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Rice Best Family 565.00
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Remaining units 0
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```
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The last hamper is abbreviated in contents and value.
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### Closing thoughts
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At a glance, there isn't a whole lot of difference between the Python and Groovy versions of this program. Both have a similar set of constructs that make handling lists and dictionaries very straightforward. Neither requires a lot of "boilerplate code" or other "ceremonial" actions.
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Also, as in the Groovy example, there is some fiddly business about being able to add units to the hamper. Basically, you pick a random position in the list of units and, starting at that position, iterate through the list until you either find a unit whose price allows it to be included or until you exhaust the list. Also, when there are only a few items left, you just toss them into the last hamper.
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Another issue worth mentioning: This isn't a particularly efficient approach. Removing elements from lists, being careless about repeated expressions, and a few other things make this less suitable for a huge redistribution problem. Still, it runs in a blink on my old machine.
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If you are shuddering at my use of `while` loops and mutating the data in this code, you probably wish I made it more functional. I couldn't think of a way to use map and reduce features in Python in conjunction with a random selection of units for repackaging. Can you?
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In the next article, I'll re-do this in Java just to see how much less effort Groovy and Python are, and future articles will cover Julia and Go.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/20/9/solve-problem-python
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作者：[Chris Hermansen][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/clhermansen
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/python_programming_question.png?itok=cOeJW-8r (Python programming language logo with question marks)
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[2]: https://opensource.com/article/20/8/solving-problem-groovy
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[3]: https://groovy-lang.org/
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[4]: https://www.python.org/
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