Python vs C#: Queries

One of the most beloved C# 3.0 features is Linq. Linq brings great power to C#, it allows you to easily write structured queries over collections or remote data sources. Now with C# is possible to make queries as easy as with other languages like Python. I decided to compare the way you make queries with C# and with Python. I found a great page showing 101 Linq examples, I decided to write Python versions of this examples. Which version do you like more?

Where – Simple 1

C# version:

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
var lowNums = from n in numbers where n < 5 select n;

Python version:

numbers = [5, 4, 1, 3, 9, 8, 6, 7, 2, 0]
low_nums = (n for n in numbers if n < 5)

 

Where – Indexed

C# version:

string[] digits = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };
var shortDigits = digits.Where((digit, index) => digit.Length < index);

Python version:

digits = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']
short_digits = (digit for index, digit in enumerate(digits) if len(digit) < index)

 

Select – Simple 1

C# version:

var numsPlusOne = from n in numbers select n + 1;

Python version:

nums_plus_one = (n + 1 for n in numbers)

 

Select – Anonymous Types 1

C# version:

string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };

var upperLowerWords =
    from w in words
    select new {Upper = w.ToUpper(), Lower = w.ToLower()};

Python version:

The exact Python version would be something like:

words = ['aPPLE', 'BlUeBeRrY', 'cHeRry']

upper_lower_words = ( type('', (), {'upper': w.upper(), 'lower': w.upper() })
                      for w in words)

But I feel more Pythonic this:

upper_lower_words = ( (w.lower(), w.upper()) for w in words)

Or even this:

upper_lower_words = ( {'upper': w.upper(), 'lower': w.upper() }
                      for w in words)

SelectMany – Compound from 1

C# version:

int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
int[] numbersB = { 1, 3, 5, 7, 8 };

var pairs =
    from a in numbersA,
         b in numbersB
    where a < b
    select new {a, b};

Python version:

numbersA = [0, 2, 4, 5, 6, 8, 9]
numbersB = [1, 3, 5, 7, 8 ]

pairs = ( (a, b) for a in numbersA 
                 for b in numbersB 
                 if a < b)

SelectMany – from Assignment

C# version:

var orders = from c in customers,
                  o in c.Orders,
                  total = o.Total
             where total >= 2000.0M
             select new {c.CustomerID, o.OrderID, total};

Python version:

I couldn’t find how to make the assignment in Python, so the version is:

orders = ( {'customer_id': c.customer_id,
            'order_id': o.order_id,
            'total': o.total }
           for c in customers
           for o in c.orders
           if o.total > 2000)

SelectMany – Multiple from

C# version:

var orders = from c in customers
             where c.Region == "WA"
             from o in c.Orders
             where o.OrderDate >= cutoffDate
             select new {c.CustomerID, o.OrderID};

Python version:

orders = ( (c.customer_id, o.order_id)
           for c in customers if c.region == 'WA'
           for o in c.orders if o.date >= cutoff_date)

Take Simple

C# version:

var first3Numbers = numbers.Take(3);

Python version:

if we are working with something like a list, we could do:

first_3_numbers = numbers[:3]

but, if we are working with iterators, we must do:

first_3_numbers = itertools.islice(numbers, None, 3)

Skip – Simple

C# version:

var allButFirst4Numbers = numbers.Skip(4);

Python version:

all_but_fist_4_numbers = numbers[4:] # list version all_but_fist_4_numbers = itertools.islice(numbers, 4, None) # iterator version 

TakeWhile – Simple

C# version:

var firstNumbersLessThan6 = numbers.TakeWhile(n => n < 6);

Python version:

fist_numbers_less_that_6 = itertools.takewhile(lambda x: x < 6, numbers)

SkipWhile – Simple

C# version:

var allButFirst3Numbers = numbers.SkipWhile(n => n % 3 != 0);

Python version:

all_but_first_3_numbers = itertools.dropwhile(lambda x: x % 3 != 0, numbers)

First & Last

C# version:

numbers.First()
numbers.Last()

Python version:

numbers[0]  # first for a list numbers[-1] # last for a list 
numbers.next()   # first for iterator list(numbers)[0] # first for iterator 
list(numbers)[-1] # last for iterator 

First – Indexed

C# version:

int evenNum = numbers.First((num, index) => (num % 2 == 0) && (index % 2 == 0));

Python version:

even_num = [n for i, n in enumerate(numbers) if n%2 == 0 and i%2 == 0][0]

or:

even_num = (n for i, n in enumerate(numbers) if n%2 == 0 and i%2 == 0).next()

to be continued…

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