Generator in Python

Things on this page are fragmentary and immature notes/thoughts of the author. Please read with your own judgement!

sum(i for i in range(10))

45

set(i for i in range(10))

{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

[i for i in range(10)]

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

list(i for i in range(10))

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

{i for i in range(10)}

{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

Must use () to get a generator. Exception is raised without ().

# this is NOT tuple!!!
(i for i in range(10))

<generator object <genexpr> at 0x7f2f84630938>

i for i in range(10)

  File "<ipython-input-1-b07d337502b5>", line 1
    i for i in range(10)
        ^
SyntaxError: invalid syntax

import itertools as it


def fibonacci(x1, x2):
    while True:
        yield x1
        x1, x2 = x2, x1 + x2


fiter = fibonacci(1, 1)
list(it.islice(fiter, 0, 10))

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

fiter = fibonacci(1, 1)
list(it.takewhile(lambda x: x < 100, fiter))

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

Comment¶

yield cannot be used recursively. Do NOT use iterator to mimic generator as iterator is not lazy evaluation.

def fib2(x1, x2):
    yield x1
    return fibonacci(x2, x1 + x2)

fiter2 = fib2(1, 1)
list(it.islice(fiter2, 0, 10))

[1]

import itertools as it


def fib3(x1, x2):
    it.chain([x1], fib3(x2, x1 + x2))

fib3(1, 1)

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-35-800beaf116b0> in <module>()
----> 1 fib3(1, 1)

<ipython-input-34-91b9f12414a5> in fib3(x1, x2)
      2 
      3 def fib3(x1, x2):
----> 4     it.chain([x1], fib3(x2, x1 + x2))

... last 1 frames repeated, from the frame below ...

<ipython-input-34-91b9f12414a5> in fib3(x1, x2)
      2 
      3 def fib3(x1, x2):
----> 4     it.chain([x1], fib3(x2, x1 + x2))

RuntimeError: maximum recursion depth exceeded