[python/en] Set, dict, and generator comprehensions (#2298)

* Add set and dict comprehensions for python 2 and 3

* Clean up formatting and generator explanation

* Include documentation for generator comprehensions

python.html.markdown

@@ -8,21 +8,21 @@ contributors:
 filename: learnpython.py
 ---
 
-Python was created by Guido Van Rossum in the early 90s. It is now one of the 
+Python was created by Guido Van Rossum in the early 90s. It is now one of the
-most popular languages in existence. I fell in love with Python for its 
+most popular languages in existence. I fell in love with Python for its
 syntactic clarity. It's basically executable pseudocode.
 
-Feedback would be highly appreciated! You can reach me at [@louiedinh](http://twitter.com/louiedinh) 
+Feedback would be highly appreciated! You can reach me at [@louiedinh](http://twitter.com/louiedinh)
 or louiedinh [at] [google's email service]
 
-Note: This article applies to Python 2.7 specifically, but should be applicable 
+Note: This article applies to Python 2.7 specifically, but should be applicable
-to Python 2.x. Python 2.7 is reaching end of life and will stop being 
+to Python 2.x. Python 2.7 is reaching end of life and will stop being
-maintained in 2020, it is though recommended to start learning Python with 
+maintained in 2020, it is though recommended to start learning Python with
 Python 3. For Python 3.x, take a look at the [Python 3 tutorial](http://learnxinyminutes.com/docs/python3/).
 
-It is also possible to write Python code which is compatible with Python 2.7 
+It is also possible to write Python code which is compatible with Python 2.7
 and 3.x at the same time, using Python [`__future__` imports](https://docs.python.org/2/library/__future__.html). `__future__` imports
-allow you to write Python 3 code that will run on Python 2, so check out the 
+allow you to write Python 3 code that will run on Python 2, so check out the
 Python 3 tutorial.
 
 ```python
@@ -549,6 +549,10 @@ filter(lambda x: x > 5, [3, 4, 5, 6, 7])   # => [6, 7]
 [add_10(i) for i in [1, 2, 3]]  # => [11, 12, 13]
 [x for x in [3, 4, 5, 6, 7] if x > 5]   # => [6, 7]
 
+# You can construct set and dict comprehensions as well.
+{x for x in 'abcddeef' if x in 'abc'}  # => {'d', 'e', 'f'}
+{x: x**2 for x in range(5)}  # => {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
+
 
 ####################################################
 ## 5. Classes
@@ -668,10 +672,10 @@ import math
 dir(math)
 
 # If you have a Python script named math.py in the same
-# folder as your current script, the file math.py will 
+# folder as your current script, the file math.py will
-# be loaded instead of the built-in Python module. 
+# be loaded instead of the built-in Python module.
 # This happens because the local folder has priority
-# over Python's built-in libraries. 
+# over Python's built-in libraries.
 
 
 ####################################################
@@ -679,44 +683,54 @@ dir(math)
 ####################################################
 
 # Generators
-# A generator "generates" values as they are requested instead of storing 
+# A generator "generates" values as they are requested instead of storing
 # everything up front
 
-# The following method (*NOT* a generator) will double all values and store it 
+# The following method (*NOT* a generator) will double all values and store it
 # in `double_arr`. For large size of iterables, that might get huge!
 def double_numbers(iterable):
     double_arr = []
     for i in iterable:
         double_arr.append(i + i)
 
-# Running the following would mean we'll double all values first and return all 
+# Running the following would mean we'll double all values first and return all
 # of them back to be checked by our condition
 for value in double_numbers(range(1000000)):  # `test_non_generator`
     print value
     if value > 5:
         break
 
-# We could instead use a generator to "generate" the doubled value as the item 
+# We could instead use a generator to "generate" the doubled value as the item
 # is being requested
 def double_numbers_generator(iterable):
     for i in iterable:
         yield i + i
 
 # Running the same code as before, but with a generator, now allows us to iterate
-# over the values and doubling them one by one as they are being consumed by 
+# over the values and doubling them one by one as they are being consumed by
-# our logic. Hence as soon as we see a value > 5, we stop break out of the 
+# our logic. Hence as soon as we see a value > 5, we break out of the
 # loop and don't need to double most of the values sent in (MUCH FASTER!)
 for value in double_numbers_generator(xrange(1000000)):  # `test_generator`
     print value
     if value > 5:
         break
 
-# BTW: did you notice the use of `range` in `test_non_generator` and `xrange` in `test_generator`? 
+# BTW: did you notice the use of `range` in `test_non_generator` and `xrange` in `test_generator`?
 # Just as `double_numbers_generator` is the generator version of `double_numbers`
 # We have `xrange` as the generator version of `range`
 # `range` would return back and array with 1000000 values for us to use
 # `xrange` would generate 1000000 values for us as we request / iterate over those items
 
+# Just as you can create a list comprehension, you can create generator
+# comprehensions as well.
+values = (-x for x in [1,2,3,4,5])
+for x in values:
+    print(x)  # prints -1 -2 -3 -4 -5 to console/terminal
+
+# You can also cast a generator comprehension directly to a list.
+values = (-x for x in [1,2,3,4,5])
+gen_to_list = list(values)
+print(gen_to_list)  # => [-1, -2, -3, -4, -5]
 
 
 # Decorators

python3.html.markdown

@@ -601,6 +601,10 @@ list(filter(lambda x: x > 5, [3, 4, 5, 6, 7]))  # => [6, 7]
 [add_10(i) for i in [1, 2, 3]]         # => [11, 12, 13]
 [x for x in [3, 4, 5, 6, 7] if x > 5]  # => [6, 7]
 
+# You can construct set and dict comprehensions as well.
+{x for x in 'abcddeef' if x in 'abc'}  # => {'d', 'e', 'f'}
+{x: x**2 for x in range(5)}  # => {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
+
 
 ####################################################
 ## 5. Modules
@@ -816,7 +820,7 @@ if __name__ == '__main__':
     sup.age = 100
     print(sup.age)
 
-    # Inherited attribute from 2nd ancestor whose default value was overriden
+    # Inherited attribute from 2nd ancestor whose default value was overridden.
     print('Can I fly? ' + str(sup.fly))
 
 
@@ -825,29 +829,35 @@ if __name__ == '__main__':
 ## 7. Advanced
 ####################################################
 
-# Generators help you make lazy code
+# Generators help you make lazy code.
 def double_numbers(iterable):
     for i in iterable:
         yield i + i
 
-# A generator creates values on the fly.
+# Generators are memory-efficient because they only load the data needed to
-# Instead of generating and returning all values at once it creates one in each
+# process the next value in the iterable. This allows them to perform
-# iteration.  This means values bigger than 15 wont be processed in
+# operations on otherwise prohibitively large value ranges.
-# double_numbers.
+# NOTE: `range` replaces `xrange` in Python 3.
-# We use a trailing underscore in variable names when we want to use a name that
+for i in double_numbers(range(1, 900000000)):  # `range` is a generator.
-# would normally collide with a python keyword
-range_ = range(1, 900000000)
-# will double all numbers until a result >=30 found
-for i in double_numbers(range_):
     print(i)
     if i >= 30:
         break
 
+# Just as you can create a list comprehension, you can create generator
+# comprehensions as well.
+values = (-x for x in [1,2,3,4,5])
+for x in values:
+    print(x)  # prints -1 -2 -3 -4 -5 to console/terminal
+
+# You can also cast a generator comprehension directly to a list.
+values = (-x for x in [1,2,3,4,5])
+gen_to_list = list(values)
+print(gen_to_list)  # => [-1, -2, -3, -4, -5]
+
 
 # Decorators
-# in this example beg wraps say
+# In this example `beg` wraps `say`. If say_please is True then it
-# Beg will call say. If say_please is True then it will change the returned
+# will change the returned message.
-# message
 from functools import wraps