Python generators code tutorial:
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import string import inspect # Informal tutorial on the generators. We will cover very briefly: ## 1) What are the generators (PEP 255). ## 2) What is yield (PEP268). # Generator in Python is a fancy object that maintain its state between calls. # In simple words generator knows what to return you next on a next function call. # And it doesn't need to store it in memory. # Think of it as a computation on-demand. Lazy, yeah. # Generator uses iterator protocol. # Thus it is common to say that every generator is an iterator. # It just means we can get a next value from the generator via next() call. # You get a generator by asking to return one from a function. # Theoretically, you can compute something blah blah to # infinity using generators, and have very little memory footprint. # The 'yield' keywoard in a function will cause the function # instead of a value return a generator-iterator object. # Lets demonstrate! # lets define a function that will return us a generator-iterator object # when we call it def english_alphabet_generator_function(): for char in string.ascii_lowercase: yield char print english_alphabet_generator_function # <function alphabet_generator at 0x1004c7320>, it is still a function # Lets call english_alphabet_generator to get the actual generator object alphabet_generator = english_alphabet_generator_function() # calling the generator function will return a generator-iterator object. print type(alphabet_generator) # <type 'generator'>, not a function anymore, we got generator object # Now lets examine function locals - varibales defined in the function namespace print alphabet_generator.gi_frame.f_locals # {}, nothing yet # Now we can use iterator protocol and get print a few characters by calling generator's next() method. print alphabet_generator.next() # prints 'a' print alphabet_generator.next() # prints 'b' print alphabet_generator.next() # prints 'c' print alphabet_generator.gi_frame.f_locals # prints {'char': 'c'} # lets exauhst our generator and see what will happen # it will print characters from d to y and throw 'StopIteration' exception empty = False while not empty: try: print alphabet_generator.next() except(StopIteration) as exp: empty = True # Another not very useful example: def _my_gen(): for x in xrange(200, 400): yield 1 gen = _my_gen() print("Sum:") print(sum(gen)) # will print 200! :-) # Please, refer to # https://jeffknupp.com/blog/2013/04/07/improve-your-python-yield-and-generators-explained/ for a in-deep explanation with # very interesting examples. Thank you! |
