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- Generators
- yield
- yield from

A **generator function** uses `yield`

instead of `return`

:

```
def evens():
num = 0
while num < 10:
yield num
num += 2
```

A **generator** is a type of iterator that yields results from a generator function.

Just call the generator function to get back a generator:

```
evengen = evens()
next(evengen) # 0
next(evengen) # 2
next(evengen) # 4
next(evengen) # 6
next(evengen) # 8
next(evengen) # ❌ StopIteration exception
```

```
def evens():
num = 0
while num < 2:
yield num
num += 2
gen = evens()
next(gen)
next(gen)
```

- When the function is called, Python immediately returns an iterator without entering the function.
- When
`next()`

is called on the iterator, it executes the body of the generator from the last stopping point up to the next`yield`

statement. - If it finds a
`yield`

statement, it pauses on the next statement and returns the value of the yielded expression. - If it doesn't reach a yield statement, it stops at the end of the function and raises a
`StopIteration`

exception.

We can use for loops on generators, since generators are just special types of iterators.

```
def evens(start, end):
num = start + (start % 2)
while num < end:
yield num
num += 2
for num in evens(12, 60):
print(num)
```

Looks a lot like...

```
evens = [num for num in range(12, 60) if num % 2 == 0]
# Or = filter(lambda x: x % 2 == 0, range(12, 60))
for num in evens:
print(num)
```

Generators are lazy: they only generate the next item when needed.

Why generate the whole sequence...

```
def find_matches(filename, match):
matched = []
for line in open(filename):
if line.find(match) > -1:
matched.append(line)
return matched
matched_lines = find_matches('frankenstein.txt', "!")
matched_lines[0]
matched_lines[1]
```

...if you only want some elements?

```
def find_matches(filename, match):
for line in open(filename):
if line.find(match) > -1:
yield line
line_iter = find_matches('frankenstein.txt', "!")
next(line_iter)
next(line_iter)
```

A large list can cause your program to run out of memory!

```
def countdown(n):
"""
Generate a countdown of numbers from N down to 'blast off!'.
>>> c = countdown(3)
>>> next(c)
3
>>> next(c)
2
>>> next(c)
1
>>> next(c)
'blast off!'
"""
```

```
def countdown(n):
"""
Generate a countdown of numbers from N down to 'blast off!'.
>>> c = countdown(3)
>>> next(c)
3
>>> next(c)
2
>>> next(c)
1
>>> next(c)
'blast off!'
"""
while n > 0:
yield n
n -= 1
yield "blast off!"
```

Let's transform this function...

```
def virfib(n):
"""Compute the nth Virahanka-Fibonacci number, for N >= 1.
>>> virfib(6)
8
"""
prev = 0 # First Fibonacci number
curr = 1 # Second Fibonacci number
k = 1
while k < n:
(prev, curr) = (curr, prev + curr)
k += 1
return curr
```

..into a generator function!

```
def generate_virfib():
"""Generate the next Virahanka-Fibonacci number.
>>> g = generate_virfib()
>>> next(g)
0
>>> next(g)
1
>>> next(g)
1
>>> next(g)
2
"""
```

```
def generate_virfib():
"""Generate the next Virahanka-Fibonacci number.
>>> g = generate_virfib()
>>> next(g)
0
>>> next(g)
1
>>> next(g)
1
>>> next(g)
2
>>> next(g)
3
"""
prev = 0 # First Fibonacci number
curr = 1 # Second Fibonacci number
while True:
yield prev
(prev, curr) = (curr, prev + curr)
```

A `yield from`

statement can be used to yield the values from an iterable one at a time.

Instead of...

```
def a_then_b(a, b):
for item in a:
yield item
for item in b:
yield item
list(a_then_b(["Apples", "Aardvarks"], ["Bananas", "BEARS"]))
```

We can write...

```
def a_then_b(a, b):
yield from a
yield from b
list(a_then_b(["Apples", "Aardvarks"], ["Bananas", "BEARS"]))
```

A `yield from`

can also yield the results of another generator function
(which could be itself).

```
def countdown(k):
if k > 0:
yield k
yield from countdown(k - 1)
```

Calls | Executed code | Bindings | Yields | |
---|---|---|---|---|

>>> c = countdown(3) | def countdown(k): | k = 3 | ||

>>> next(c) | if k > 0: | |||

yield k | 3 | |||

>>> next(c) | yield from countdown(k - 1) | |||

def countdown(k): | k = 2 | |||

if k > 0: | ||||

yield k | 2 | |||

>>> next(c) | yield from countdown(k - 1) | |||

def countdown(k): | k = 1 | |||

if k > 0: | ||||

yield k | 1 | |||

>>> next(c) | yield from countdown(k - 1) | |||

def countdown(k): | k = 0 | |||

if k > 0: | ||||

yield k | ||||

yield from countdown(k - 1) | ||||

StopIteration |

When a generator function executes a return statement, it exits and *cannot* yield more values.

```
def f(x):
yield x
yield x + 1
return
yield x + 3
```

```
list(f(2)) # [2, 3]
```

Python allows you to specify a value to be returned,
but this value is *not* yielded.

```
def g(x):
yield x
yield x + 1
return x + 2
yield x + 3
```

```
list(g(2)) # [2, 3]
```

It is possible to access that return value, with this one weird trick. But you won't ever need this in 61A!

```
def h(x):
y = yield from g(x)
yield y
```

```
list(h(2)) # [2, 3, 4]
```

The number of partitions of a positive integer `n`

, using parts up to size `m`

, is the number
of ways in which `n`

can be expressed as the sum of positive integer parts up to `m`

in
increasing order.

`count_partitions(6, 4)`

2 + 4 = 6

1 + 1 + 4 = 6

3 + 3 = 6

1 + 2 + 3 = 6

1 + 1 + 1 + 3 = 6

2 + 2 + 2 = 6

1 + 1 + 2 + 2 = 6

1 + 1 + 1 + 1 + 2 = 6

1 + 1 + 1 + 1 + 1 + 1 = 6

```
def count_partitions(n, m):
"""
>>> count_partitions(6, 4)
9
"""
if n < 0 or m == 0:
return 0
else:
exact_match = 0
if n == m:
exact_match = 1
with_m = count_partitions(n-m, m)
without_m = count_partitions(n, m-1)
return exact_match + with_m + without_m
```

Each call to the generator should yield a partition.

```
def partitions(n, m):
"""List partitions.
>>> for p in partitions(6, 4): print(p)
4 + 2
4 + 1 + 1
3 + 3
3 + 2 + 1
3 + 1 + 1 + 1
2 + 2 + 2
2 + 2 + 1 + 1
2 + 1 + 1 + 1 + 1
1 + 1 + 1 + 1 + 1 + 1
"""
```

```
def partitions(n, m):
"""List partitions.
>>> for p in partitions(6, 4): print(p)
4 + 2
4 + 1 + 1
3 + 3
3 + 2 + 1
3 + 1 + 1 + 1
2 + 2 + 2
2 + 2 + 1 + 1
2 + 1 + 1 + 1 + 1
1 + 1 + 1 + 1 + 1 + 1
"""
if n < 0 or m == 0:
return
else:
if n == m:
yield str(m)
for p in partitions(n-m, m):
yield str(m) + " + " + p
yield from partitions(n, m - 1)
```