David Lord <david...@gmail.com> added the comment:
Is this performance issue supposed to be fixed in 3.9? I'm still observing
severe slowdown by inheriting from `Generic[T]`.
I'm currently adding typing to Werkzeug, where we define many custom data
structures such as `MultiDict`. It would be ideal for these classes to be
recognized as generic mappings. I remembered hearing about this performance
issue somewhere, so I decided to test what happens.
Here's a minimal example without Werkzeug, the results in Werkzeug are similar
or worse. I'd estimate each request creates about 10 of the various data
structures, which are then accessed by user code, so I simulated that by
creating and iterating a list of objects.
```python
class Test:
def __init__(self, value):
self.value = value
def main():
ts = [Test(x) for x in range(10)]
sum(t.value for t in ts)
```
```
$ python3.9 -m timeit -n 100000 -s 'from example import main' 'main()'
100000 loops, best of 5: 7.67 usec per loop
```
```python
import typing
V = typing.TypeVar("V")
class Test(typing.Generic[V]):
def __init__(self, value: V) -> None:
self.value = value
def main():
ts = [Test(x) for x in range(10)]
sum(t.value for t in ts)
```
```
$ python3.9 -m timeit -n 100000 -s 'from example import main' 'main()'
100000 loops, best of 5: 18.2 usec per loop
```
There is more than a 2x slowdown when using `Generic`. The timings (7 vs 18
usec) are the same across Python 3.6, 3.7, 3.8, and 3.9. It seems that 3.9 does
not fix the performance issue.
Since we currently support Python 3.6+, I probably won't be able to use
generics anyway due to the performance in those versions, but I wanted to make
sure I'm not missing something with 3.9.
----------
nosy: +davidism
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<https://bugs.python.org/issue39168>
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