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Diffstat (limited to 'python.html.markdown')
| -rw-r--r-- | python.html.markdown | 93 |
1 files changed, 65 insertions, 28 deletions
diff --git a/python.html.markdown b/python.html.markdown index 28b0a7ae..55f56071 100644 --- a/python.html.markdown +++ b/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 -most popular languages in existence. I fell in love with Python for its +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 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 -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 +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 +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 @@ -126,6 +126,9 @@ not False # => True # A string can be treated like a list of characters "This is a string"[0] # => 'T' +# You can find the length of a string +len("This is a string") # => 16 + #String formatting with % #Even though the % string operator will be deprecated on Python 3.1 and removed #later at some time, it may still be good to know how it works. @@ -298,6 +301,9 @@ filled_dict.keys() # => ["three", "two", "one"] filled_dict.values() # => [3, 2, 1] # Note - Same as above regarding key ordering. +# Get all key-value pairs as a list of tuples with "items()" +filled_dicts.items() # => [("one", 1), ("two", 2), ("three", 3)] + # Check for existence of keys in a dictionary with "in" "one" in filled_dict # => True 1 in filled_dict # => False @@ -543,6 +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 @@ -662,39 +672,66 @@ 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 -# be loaded instead of the built-in Python module. +# folder as your current script, the file math.py will +# 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. #################################################### ## 7. Advanced #################################################### -# Generators help you make lazy code +# Generators +# 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 +# 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 +# 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 +# is being requested +def double_numbers_generator(iterable): for i in iterable: yield i + i -# A generator creates values on the fly. -# Instead of generating and returning all values at once it creates one in each -# iteration. This means values bigger than 15 wont be processed in -# double_numbers. -# Note xrange is a generator that does the same thing range does. -# Creating a list 1-900000000 would take lot of time and space to be made. -# xrange creates an xrange generator object instead of creating the entire list -# like range does. -# We use a trailing underscore in variable names when we want to use a name that -# would normally collide with a python keyword -xrange_ = xrange(1, 900000000) - -# will double all numbers until a result >=30 found -for i in double_numbers(xrange_): - print i - if i >= 30: +# 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 +# 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`? +# 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 # in this example beg wraps say |