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Satwik Kansal 2018-02-26 00:34:22 +05:30
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@ -367,6 +367,7 @@ for i, some_dict[i] in enumerate(some_string):
### ▶ Evaluation time discrepancy ### ▶ Evaluation time discrepancy
1\.
```py ```py
array = [1, 8, 15] array = [1, 8, 15]
g = (x for x in array if array.count(x) > 0) g = (x for x in array if array.count(x) > 0)
@ -379,28 +380,21 @@ array = [2, 8, 22]
[8] [8]
``` ```
#### 💡 Explanation 2\.
- In a [generator](https://wiki.python.org/moin/Generators) expression, the `in` clause is evaluated at declaration time, but the conditional clause is evaluated at runtime.
- So before runtime, `array` is re-assigned to the list `[2, 8, 22]`, and since out of `1`, `8` and `15`, only the count of `8` is greater than `0`, the generator only yields `8`.
---
### ▶ Generator with Slice Assignment
```py ```py
iter1 = [1,2,3,4] array_1 = [1,2,3,4]
g1 = (x for x in iter1) g1 = (x for x in array_1)
iter1 = [1,2,3,4,5] array_1 = [1,2,3,4,5]
iter2 = [1,2,3,4] array_2 = [1,2,3,4]
g2 = (x for x in iter2) g2 = (x for x in array_2)
iter2[:] = [1,2,3,4,5] array_2[:] = [1,2,3,4,5]
``` ```
**Output:** **Output:**
```py ```py
>>>print(list(g1)) >>> print(list(g1))
[1,2,3,4] [1,2,3,4]
>>> print(list(g2)) >>> print(list(g2))
@ -409,11 +403,11 @@ iter2[:] = [1,2,3,4,5]
#### 💡 Explanation #### 💡 Explanation
- In the first expression `g1` yields the elements of original list `iter1`(instead of updated one). Since the `in` - In a [generator](https://wiki.python.org/moin/Generators) expression, the `in` clause is evaluated at declaration time, but the conditional clause is evaluated at runtime.
clause is evaluated at the time of generator declaration, hence generator expression `g1` still have reference of original list - So before runtime, `array` is re-assigned to the list `[2, 8, 22]`, and since out of `1`, `8` and `15`, only the count of `8` is greater than `0`, the generator only yields `8`.
`iter1` - The differences in the output of `g1` and `g2` in the second part is due the way variables `array_1` and `array_2` are re-assigned values.
- In the first case, `array_1` is binded to the new object `[1,2,3,4,5]` and since the `in` clause is evaluated at the declaration time it still refers to the old object `[1,2,3,4]` (which is not destroyed).
- In the second example, list `iter1` is updated using slice assignment. Slice assignment, in contrast to normal assignment(which creates a new list), updates the same list. We can view this as the slice of list being replace by the iterable on right hand side of equality. Hence, the generator `g2` yields updated `iter2` elements. - In the second case, the slice assignment to `array_2` updates the same old object `[1,2,3,4]` to `[1,2,3,4,5]`. Hence both the `g2` and `array_2` still have reference to the same object (which has now been updated to `[1,2,3,4,5]`).
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