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Merge pull request #138 from scidam/master
New snippet: yielding from... return!
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README.md
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README.md
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@ -69,6 +69,7 @@ So, here we go...
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- [▶ Be careful with chained operations](#-be-careful-with-chained-operations)
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- [▶ Name resolution ignoring class scope](#-name-resolution-ignoring-class-scope)
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- [▶ Needle in a Haystack](#-needle-in-a-haystack)
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- [▶ Yielding from... return!](#-yielding-from-return)
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- [Section: The Hidden treasures!](#section-the-hidden-treasures)
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- [▶ Okay Python, Can you make me fly? *](#-okay-python-can-you-make-me-fly-)
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- [▶ `goto`, but why? *](#-goto-but-why-)
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@ -1883,6 +1884,64 @@ tuple()
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---
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### ▶ Yielding from... return!
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1\.
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```py
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def func(k):
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if k == 3:
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return ["A string..."]
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else:
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yield from range(k)
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```
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**Output:**
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```py
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>>> list(func(3)) # expected: ["A string..."]
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[]
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```
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The same behavior is true if we rewrite `yield from` as a for loop.
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2\.
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```py
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def func(k):
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if k == 3:
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return ["A string..."]
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else:
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for j in range(k):
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yield j
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```
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**Output:**
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```py
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>>> list(func(3)) # expected: ["A string..."]
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[]
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```
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#### 💡 Explanation:
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Starting from Python 3.3+ it became possible to use return statement
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with values inside generators ([PEP380](https://www.python.org/dev/peps/pep-0380/)). The [official doc](https://www.python.org/dev/peps/pep-0380/#enhancements-to-stopiteration) says that "... `return expr` in a generator causes `StopIteration(expr)` to be raised upon exit from the generator."
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So, to get `["A string..."]` from the generator `func` we need to catch `StopIteration`, e.g.:
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```py
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try:
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next(func(3))
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except StopIteration as e:
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string = e.value
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```
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```py
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>>> string
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['A string...']
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```
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Note that `list(...)` automatically catches `StopIteration`. In case of `func(3)` `StopIteration` raises at the beginning because of `return` statement. Therefore, `list(func(3))` results in an empty list.
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---
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---
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