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Add example: For what?

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* Adds a new example
* Merges an existing example "Loop variable resilient to changes"
  in the explanation of this example

Closes https://github.com/satwikkansal/wtfpython/issues/23
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Satwik Kansal 2017-09-05 00:52:19 +05:30
parent 7e93e0cb91
commit 753d0b6aae
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@ -85,9 +85,9 @@ So, here ya go...
- [💡 Explanation:](#-explanation-19)
- [Needle in a Haystack](#needle-in-a-haystack)
- [💡 Explanation:](#-explanation-20)
- [not knot!](#not-knot)
- [For what?](#for-what)
- [💡 Explanation:](#-explanation-21)
- [Loop variable resilient to changes](#loop-variable-resilient-to-changes)
- [not knot!](#not-knot)
- [💡 Explanation:](#-explanation-22)
- [Let's see if you can guess this?](#lets-see-if-you-can-guess-this)
- [💡 Explanation:](#-explanation-23)
@ -1481,6 +1481,61 @@ tuple()
---
### For what?
Suggested by @MittalAshok in [this](https://github.com/satwikkansal/wtfpython/issues/23) issue.
```py
some_string = "wtf"
some_dict = {}
for i, some_dict[i] in enumerate(some_string):
pass
```
**Outuput:**
```py
>>> some_dict # An indexed dict is created.
{0: 'w', 1: 'f', 2: 'f'}
```
#### 💡 Explanation:
* A `for` statement is defined in the [Python grammar](https://docs.python.org/3/reference/grammar.html) as:
```
for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]
```
Where `exprlist` is the assignment target. This means that the equivalent of `{exprlist} = {next_value}` is **executed for each item** in the iterable.
An interesting example suggested by @tukkek in [this](https://github.com/satwikkansal/wtfPython/issues/11) issue illustrates this:
```py
for i in range(4):
print(i)
i = 10
```
**Output:**
```
0
1
2
3
```
Did you expect the loop to run just once?
**💡 Explanation:**
- The assignment statement `i = 10` never affects the iterations of the loop because of the way for loops work in Python. Before the beginning of every iteration, the next item provided by the iterator (`range(4)` this case) is unpacked and assigned the target list variables (`i` in this case).
* The `enumerate(some_string)` function yields a new value `i` (A counter going up) and a character from the `some_string` in each iteration. It then sets the (just assigned) `i` key of the dictionary `some_dict` to that character. The unrolling of the loop can be simplified as:
```py
>>> i, some_dict[i] = (0, 'w')
>>> i, some_dict[i] = (1, 't')
>>> i, some_dict[i] = (2, 'f')
>>> some_dict
```
---
### not knot!
Suggested by @MostAwesomeDude in [this](https://github.com/satwikkansal/wtfPython/issues/1) issue.
@ -1510,36 +1565,6 @@ SyntaxError: invalid syntax
---
### Loop variable resilient to changes
Suggested by @tukkek in [this](https://github.com/satwikkansal/wtfPython/issues/11) issue.
```py
for i in range(7):
print(i)
i = 10
```
**Output:**
```
0
1
2
3
4
5
6
```
Did you expect the loop to run just once?
#### 💡 Explanation:
- [Source](https://docs.python.org/3/reference/compound_stmts.html#the-for-statement)
- The assignment statement `i = 10` never affects the iterations of the loop because of the way for loops work in Python. Before the beginning of every iteration, the next item provided by the iterator (`range(7)` this case) is unpacked and assigned the target list variables (`i` in this case).
---
### Let's see if you can guess this?
Suggested by @PiaFraus in [this](https://github.com/satwikkansal/wtfPython/issues/9) issue.