PERF: added no exception versions of '_string_to_dts' and 'parse_iso_8601_datetime' functions
- [x] closes #N/A
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

`asv continuous -f 1.05 origin/master HEAD -b ^io.csv -b ^timeseries -a warmup_time=1 -a sample_time=1`:

master| patch| ratio| test name
-|-|-|-
98.4±3ms|         76.2±2ms|     0.77|  timeseries.ToDatetimeFormatQuarters.time_infer_quarter
15.2±0.6ms|      9.75±0.04ms|     0.64|  io.csv.ReadCSVParseSpecialDate.time_read_special_date('mdY')
37.1±2ms|       22.4±0.1ms|     0.60|  io.csv.ReadCSVParseSpecialDate.time_read_special_date('mY')
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=h1) Report
> Merging [#26220](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/65466f04025474c3ee1bd2f4e49a1c5e24b76f7b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26220/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26220      +/-   ##
==========================================
- Coverage   91.97%   91.97%   -0.01%     
==========================================
  Files         175      175              
  Lines       52379    52379              
==========================================
- Hits        48178    48175       -3     
- Misses       4201     4204       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.52% <ø> (ø)` | :arrow_up: |
| #single | `40.69% <ø> (-0.16%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.71% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=footer). Last update [65466f0...0598922](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=h1) Report
> Merging [#26220](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f46ab96fc3f6fb2e764295b4922f5f5e2323d2d8?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26220/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26220      +/-   ##
==========================================
- Coverage   91.98%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52379    52374       -5     
==========================================
- Hits        48183    48174       -9     
- Misses       4196     4200       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.53% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.72% <ø> (-0.16%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `94.59% <0%> (-0.28%)` | :arrow_down: |
| [pandas/io/excel/\_util.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL191dGlsLnB5) | `87.32% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.47% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.52% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `97.96% <0%> (-0.01%)` | :arrow_down: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `90.22% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/dtypes/inference.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2luZmVyZW5jZS5weQ==) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `97.24% <0%> (+0.01%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/26220/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=footer). Last update [f46ab96...cffa8a6](https://codecov.io/gh/pandas-dev/pandas/pull/26220?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > So the speedup here is purely replacing the "except? -1" clause in the cdef with manual error checking right?

@WillAyd you're right. OK thanks. Do you know if the question mark in that statement has a performance hit? 

Not against this change just surprised to see it make that big of a difference. Certainly being able to leverage the built-in Cython functionality would be simpler and preferable if removing the question mark gets us closer (hence my question) > OK thanks. Do you know if the question mark in that statement has a performance hit?

The performance degradation from this is unimportant because most likely this is one additional `PyErr_Occurred()` call, I think.

> Not against this change just surprised to see it make that big of a difference. Certainly being able to leverage the built-in Cython functionality would be simpler and preferable if removing the question mark gets us closer (hence my question)

The main performance increase is that we stop throwing a `ValueError` exception (this is too expensive for python). So removing the question mark will not allow performance gains and will most likely change the logic of date parsing. > The main performance increase is that we stop throwing a ValueError exception (this is too expensive for python)

@anmyachev any idea how many more formats/functions you plan to optimize like this?  In the past we've been trying to get code _out_ of the C files and to stay python-idiomatic where possible.  It looks like you're making an effort to keep the non-idiomatic code contained to np_datetime, which is good.

Could the raising of ValueError be made more efficient upstream in cython?  Just spitballing: if the return-code based version is really equivalent to the raising-based version, it seems like cython should be able to figure that out. > @anmyachev any idea how many more formats/functions you plan to optimize like this? In the past we've been trying to get code _out_ of the C files and to stay python-idiomatic where possible. It looks like you're making an effort to keep the non-idiomatic code contained to np_datetime, which is good.

At the moment, nothing more in this way. This function was "hot" when profiling, and the main slowdown was just in throwing an exception. Other functions with exactly the same problem have not been identified yet.

> Could the raising of ValueError be made more efficient upstream in cython? Just spitballing: if the return-code based version is really equivalent to the raising-based version, it seems like cython should be able to figure that out.

Probably, but the main problem is the speed of exceptions in the python itself, and not how сython generates it.

In the first case, we need the function to throw an exception, and in the other it does not, depending on our choice(for this we use `want_exc` argument). I cannot achieve this purely with the construction of a cython.

If the existence of functions "_string_to_dts_noexc" and "_string_to_dts" is not very good for you, then I can try to make one function, but with an additional parameter (to throw an exception or not) while leaving with the instruction "except? -1". What do you say? > If the existence of functions "_string_to_dts_noexc" and "_string_to_dts" is not very good for you [...]

Not a problem at all.  Why would we ever use the slow version?

> Probably, but the main problem is the speed of exceptions in the python itself, and not how сython generates it.

_string_to_dts is `cdef`, so it can only be called from C/cython.  The idea was that for these functions, cython could make the substitution and only ever do the raising (slow) in a python-exposed function. > The idea was that for these functions, cython could make the substitution and only ever do the raising (slow) in a python-exposed function.

This is possible if desired, we'd just have to make `parse_iso8601` return different error codes to specify correct exception messages.

The exceptions themselves are not _that_ slow, but in this particular case they were taking way more time than `parse_iso8601` function itself (because the function on usual data very quickly either parses the string or figures out it's not a ISO8601 datetime - nothing of this requires new allocations), and raising an exception requires allocating a few objects (at least the exception and the traceback objects) and unwinding the stack to store in the traceback. If the only thing we care (as in this particular case) is whether exception happened _at all_ (and we don't care about message or stacktrace), then we should be fine using boolean "error happened" flag.

As for Cython being smart there - I think it might be possible to make a keyword or something that would tell Cython to generate some error-code-based error checking and hide it pretending it's exceptions (like `@nogil` pretends it's still Pythonic but releases GIL). > this is adding a lot of complexity for a relativily small gain. If you want to remove the exception handling, then just do it and force a check everytime it is used. having 2 function is way more complex code.

@jreback "noexc" version of functions removed @jreback I simplified changes. @jreback @WillAyd all CI builds (include `macOS py35_macos`) actually completed successfully. thanks @anmyachev  This seems to have slowed down `timestamp.TimestampConstruction.time_parse_iso8601_tz` http://pandas.pydata.org/speed/pandas/index.html#timestamp.TimestampConstruction.time_parse_iso8601_tz?commits=71207255f32af73cd701fe4ecf56a643c925a9e1-2e4e0b9058cf9f5a3ca81d2fb405fcd3b9317951 @TomAugspurger I'll see what's the problem.