A generator function creates and returns an iterator. This way of creating an iterator is often simpler than the way we did it previously.
yield
statement
StopIteration
exception
Let’s build an
iterable
that’s like a
range,
but that can iterate through fractions as well as whole numbers.
The difference
end-start
is the distance from
start
to
end.
The fraction
i/n
ranges from 0 to 1 as the
for
loop iterates.
Therefore
(end-start) * i/n
is the distance from the
start
to the next number to be printed.
The function
floatRange
returns
None.
def floatRange(start, end, n):
"Print n+1 equally spaced numbers, from start to end inclusive."
assert isinstance(n, int) and n > 0
for i in range(n + 1):
print(start + (end - start) * i / n)
floatRange(0.0, 1.0, 10)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
A function that contains a
yield
statement is called a
generator
function.
For example,
the following
floatRange
is a generator function.
It looks like the generator function executes its
for
loop and then returns nothing,
since the generator function has no
return
statement.
(More precisely, it looks like the generator function returns
None.)
But the generator function does return a value.
Moreover, the generator returns this value immediately,
even before executing the
for
loop.
The value is a certain kind of iterator called a
generator
iterator.
#Exactly the same function, except that I changed print to yield.
def floatRange(start, end, n):
"Yield n+1 equally spaced numbers, from start to end inclusive."
assert isinstance(n, int) and n > 0
for i in range(n + 1):
yield start + (end - start) * i / n
it = floatRange(0.0, 1.0, 10) #it is a generator iterator.
print(f"type(it) = {type(it)}")
#Demonstrate that it is an iterator.
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__())
print(it.__next__()) #This call to __next__ raises a StopIteration exception.
type(it) = <class 'generator'>
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Traceback (most recent call last):
File "/Users/mark/python/junk2.py", line 25, in <module>
print(it.__next__()) #This call to __next__ raises a StopIteration exception.
StopIteration
Now that we have demonstrated that the generator function
floatRange
returns an iterator,
we can give this return value to a
for
loop.
def floatRange(start, end, n):
"Yield n+1 equally spaced numbers, from start to end inclusive."
assert isinstance(n, int) and n > 0
for i in range(n + 1):
yield start + (end - start) * i / n
for f in floatRange(0.0, 1.0, 10):
print(f)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
So it’s very easy to make your own generator function. Just follow these two steps:
floatRange
that
prints
the desired sequence of values,
and then returns.
print
statement in the function to a
yield
statement.
The function
eagerFloatRange
creates and returns a new
list.
This new
list
is completely created up front,
before we begin to go around the bottom
for
loop.
def eagerFloatRange(start, end, n):
"Return a list of n+1 equally spaced floats, from start to end inclusive."
assert isinstance(n, int) and n > 0
myList = []
for i in range(n + 1):
print(f"About to append {start + (end - start) * i / n}")
myList.append(start + (end - start) * i / n)
return myList
for f in eagerFloatRange(0.0, 1.0, 10):
print(f"f = {f}")
About to append 0.0 About to append 0.1 About to append 0.2 About to append 0.3 About to append 0.4 About to append 0.5 About to append 0.6 About to append 0.7 About to append 0.8 About to append 0.9 About to append 1.0 f = 0.0 f = 0.1 f = 0.2 f = 0.3 f = 0.4 f = 0.5 f = 0.6 f = 0.7 f = 0.8 f = 0.9 f = 1.0
The following function
floatRange
does not create any
list.
It creates and returns a generator iterator.
floatRange
then goes around its
for
loop very reluctantly.
It gets stuck each time it hits the
yield
statement,
and remains stuck there until the
for
loop at the bottom of the program calls
__next__.
def floatRange(start, end, n):
"Yield n+1 equally spaced numbers, from start to end inclusive."
assert isinstance(n, int) and n > 0
for i in range(n + 1):
print(f"About to yield {start + (end - start) * i / n}")
yield start + (end - start) * i / n
for f in floatRange(0.0, 1.0, 10):
print(f"f = {f}")
About to yield 0.0 f = 0.0 About to yield 0.1 f = 0.1 About to yield 0.2 f = 0.2 About to yield 0.3 f = 0.3 About to yield 0.4 f = 0.4 About to yield 0.5 f = 0.5 About to yield 0.6 f = 0.6 About to yield 0.7 f = 0.7 About to yield 0.8 f = 0.8 About to yield 0.9 f = 0.9 About to yield 1.0 f = 1.0
A generator function saves time and memory by not creating a list.
And if we break out of the
for
loop (the
for f in floatRange)
loop prematurely with a
break
statemnt,
the generator iterator will compute only the values that are actually used
by the
for
loop.
Instead of inventing the name
floatRange,
let’s gave the generator the same name as the existing
range.
To make it possible for
floatdemo.py
to use both
ranges,
we give the generator the last name
float
by puting it in a module named
float.
One complication with having two function with the first name:
float.py
had to
import
builtins
so that it could call the
built-in function
range
(in
line
24)
as well as the
range
we created (in
line
31).
0 1 2 3 4 5 6 7 8 9 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 .7 is in the float.range.
Tue 2019-12-31 Wed 2020-01-01 Thu 2020-01-02 Fri 2020-01-03 Sat 2020-01-04 Sun 2020-01-05 Mon 2020-01-06 Tue 2020-01-07
""" useinspection.py Loop through the sequence of restaurant inspections yielded by inspection.inspections. """ import sys import inspection #the inspection.py file I wrote import datetime import textwrap for inspection in inspection.inspections(41_320_866): #Wo Hop, 17 Mott Street assert isinstance(inspection, list) #just for documentation assert len(inspection) == 26 assert int(inspection[0]) == 41_320_866 #Convert the inspection date field into an object of class datetime.date #so we can print it nicely. d = datetime.datetime.strptime(inspection[8], "%m/%d/%Y").date() print(inspection[1], d.strftime("%A, %B %-d, %Y")) #Split the violation description field into lines of at most 80 characters. lines = textwrap.wrap(inspection[11], width = 80) for line in lines: print(line) print() sys.exit(0)
WO HOP 17 Monday, June 19, 2017 Cold food item held above 41º F (smoked fish and reduced oxygen packaged foods above 38 ºF) except during necessary preparation. WO HOP 17 Monday, June 19, 2017 Non-food contact surface improperly constructed. Unacceptable material used. Non-food contact surface or equipment improperly maintained and/or not properly sealed, raised, spaced or movable to allow accessibility for cleaning on all sides, above and underneath the unit. WO HOP 17 Friday, May 25, 2018 Food not cooked to required minimum temperature. WO HOP 17 Friday, May 25, 2018 Non-food contact surface improperly constructed. Unacceptable material used. Non-food contact surface or equipment improperly maintained and/or not properly sealed, raised, spaced or movable to allow accessibility for cleaning on all sides, above and underneath the unit. WO HOP 17 Thursday, May 23, 2019 Food contact surface not properly maintained. WO HOP 17 Thursday, May 23, 2019 Non-food contact surface improperly constructed. Unacceptable material used. Non-food contact surface or equipment improperly maintained and/or not properly sealed, raised, spaced or movable to allow accessibility for cleaning on all sides, above and underneath the unit.
Try another CAMIS, e.g. 50036220 for Roof at Park South.
ROOF AT PARK SOUTH Wednesday, May 23, 2018 Cold food item held above 41º F (smoked fish and reduced oxygen packaged foods above 38 ºF) except during necessary preparation. ROOF AT PARK SOUTH Wednesday, May 23, 2018 Food not protected from potential source of contamination during storage, preparation, transportation, display or service. ROOF AT PARK SOUTH Wednesday, May 23, 2018 Sanitized equipment or utensil, including in-use food dispensing utensil, improperly used or stored. ROOF AT PARK SOUTH Friday, September 21, 2018 Food worker does not use proper utensil to eliminate bare hand contact with food that will not receive adequate additional heat treatment. ROOF AT PARK SOUTH Friday, September 21, 2018 Non-food contact surface improperly constructed. Unacceptable material used. Non-food contact surface or equipment improperly maintained and/or not properly sealed, raised, spaced or movable to allow accessibility for cleaning on all sides, above and underneath the unit. ROOF AT PARK SOUTH Saturday, May 25, 2019 Cold food item held above 41º F (smoked fish and reduced oxygen packaged foods above 38 ºF) except during necessary preparation. ROOF AT PARK SOUTH Saturday, May 25, 2019 Non-food contact surface improperly constructed. Unacceptable material used. Non-food contact surface or equipment improperly maintained and/or not properly sealed, raised, spaced or movable to allow accessibility for cleaning on all sides, above and underneath the unit.
Unfortunately, each time we call
inspection.inspections,
we have to wait for the CSV file to be downloaded from
data.cityofnewyork.us.
Make a smarter
inspection.inspections
that downloads the CSV file only the
first
time it is called,
and stores the records in a
list
of approximately 400,000 items,
each of which is a list of 18 items.
Subsequent calls to
inspection.inspections
should get their data from this list.
float.range
is a function, but
float.range(0.0, 1.0, 10)
is a
generator.
(float.range(0.0, 1.0, 10)
is the value returned by the
float.range
function when given those three arguments.)
import float print("type(float.range) =", type(float.range)) print("type(float.range(0.0, 1.0, 10)) =", type(float.range(0.0, 1.0, 10)))
type(float.range) = <class 'function'> type(float.range(0.0, 1.0, 10)) = <class 'generator'>
yield
can create a context manager
here.