= Generators =

https://wiki.python.org/moin/Generators

== Introduction ==

=== Generator Expressions ===

{{{#!python
# list comprehension
doubles = [2 * n for n in range(50)]

# same as the list comprehension above
doubles = list(2 * n for n in range(50))
}}}

=== Generator Functions ===

Before:

[[http://www.pythontutor.com/visualize.html#code=def+squares(n)%3A%0A++++result+%3D+%5B%5D%0A++++i+%3D+0%0A++++while+i%3Cn%3A%0A++++++++result.append(i*i)%0A++++++++i+%2B%3D+1%0A++++return+result%0A%0As+%3D+0%0Afor+i+in+squares(10)%3A%0A++++s+%2B%3D+i%0Aprint(s)&mode=display&cumulative=true&heapPrimitives=false&drawParentPointers=false&textReferences=false&showOnlyOutputs=false&py=3&curInstr=0|Visualize in Python Tutor]]

{{{#!python
def squares(n):
    result = []
    i = 0
    while i<n:
        result.append(i*i)
        i += 1
    return result

s = 0
for i in squares(10):
    s += i
print(s)
}}}

After:

[[http://www.pythontutor.com/visualize.html#code=def+squares(n)%3A%0A++++i+%3D+0%0A++++while+i%3Cn%3A%0A++++++++yield+i*i%0A++++++++i+%2B%3D+1%0A%0As+%3D+0%0Afor+i+in+squares(10)%3A%0A++++s+%2B%3D+i%0Aprint(s)&mode=display&cumulative=false&heapPrimitives=false&drawParentPointers=false&textReferences=false&showOnlyOutputs=false&py=3&curInstr=0|Visualize in Python Tutor]]

{{{#!python
def squares(n):
    i = 0
    while i<n:
        yield i*i
        i += 1

s = 0
for i in squares(10):
    s += i
print(s)
}}}

=== Iterator Class ===

For comparison.

{{{#!python
class squares(object):
    def __init__(self, n):
        self.n = n
        self.num = 0

    def __iter__(self):
        return self

    def next(self):
        if self.num < self.n:
            cur, self.num = self.num, self.num+1
            return cur*cur
        else:
            raise StopIteration()
}}}


== Streaming ==

[[http://flask.pocoo.org/docs/patterns/streaming/|Streaming content in Flask]]

{{{#!python
from flask import Response

@app.route('/large.csv')
def generate_large_csv():
    def generate():
        for row in iter_all_rows():
            yield ','.join(row) + '\n'
    return Response(generate(), mimetype='text/csv')
}}}