BUG: Period.strftime crashes for invalid format strings

[JozsefKutas]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
period = pd.Period("2023-Q2")
period.strftime("%Y-Q%Q")  # %Q is invalid, should be %q
# Process finished with exit code -1073740791 (0xC0000409)

Issue Description

Calling Period.strftime with an invalid format string crashes the Python session.

Expected Behavior

Ideally, an exception would be raised instead. For example, with Timestamp:

import pandas as pd
ts = pd.Timestamp("2023-06-08")
ts.strftime("%Y-Q%Q")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas\_libs\tslibs\timestamps.pyx", line 1493, in pandas._libs.tslibs.timestamps.Timestamp.strftime
ValueError: Invalid format string

Installed Versions

INSTALLED VERSIONS

commit : 965ceca
python : 3.9.7.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19044
machine : AMD64
processor : Intel64 Family 6 Model 140 Stepping 1, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United Kingdom.1252

pandas : 2.0.2
numpy : 1.24.3
pytz : 2023.3
dateutil : 2.8.2
setuptools : 57.4.0
pip : 21.2.3
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
brotli : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : None
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[tpackard1]

Hi all,

When I tried the reproducible example on main branch, I did not get an error for either of the following:

import pandas as pd
period = pd.Period("2023-Q2")
period.strftime("%Y-Q%Q") # '2023-Q%Q'

import pandas as pd
ts = pd.Timestamp("2023-06-08")
ts.strftime("%Y-Q%Q") # '2023-Q%Q'

INSTALLED VERSIONS
------------------
commit           : 3ef98e65d1ab5a7c0eb77f9bbbb1d95ecabfeabb
python           : 3.10.10.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.10.102.1-microsoft-standard-WSL2
Version          : #1 SMP Wed Mar 2 00:30:59 UTC 2022
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 2.1.0.dev0+943.g3ef98e65d1
numpy            : 1.23.5
pytz             : 2022.6
dateutil         : 2.8.2
setuptools       : 65.5.0
pip              : 22.3.1
Cython           : 0.29.35
pytest           : 7.2.0
hypothesis       : 6.61.0
sphinx           : 4.5.0
blosc            : 1.11.0
feather          : None
xlsxwriter       : 3.0.3
lxml.etree       : 4.9.2
html5lib         : 1.1
pymysql          : 1.0.2
psycopg2         : 2.9.5
jinja2           : 3.1.2
IPython          : 8.7.0
pandas_datareader: None
bs4              : 4.11.1
bottleneck       : 1.3.5
brotli           :
fastparquet      : 2022.12.0
fsspec           : 2021.11.0
gcsfs            : 2021.11.0
matplotlib       : 3.6.2
numba            : 0.56.4
numexpr          : 2.8.4
odfpy            : None
openpyxl         : 3.0.10
pandas_gbq       : None
pyarrow          : 9.0.0
pyreadstat       : 1.2.0
pyxlsb           : 1.0.10
s3fs             : 2021.11.0
scipy            : 1.9.3
snappy           :
sqlalchemy       : 1.4.45
tables           : 3.7.0
tabulate         : 0.9.0
xarray           : 2022.12.0
xlrd             : 2.0.1
zstandard        : 0.19.0
tzdata           : 2022.7
qtpy             : None
pyqt5            : None

I did however get the same behavior as @JozsefKutas describes when I tried the reproducible example with my latest version of pandas so I think this is a valid Bug but I might open a separate Issue regarding the behavior on main that I got and I will update this comment when I do.

[Aloqeely]

Still crashes for me on main. cc @MarcoGorelli
The crash seems to be happening in these lines (code enters strftime then exits)

pandas/pandas/_libs/tslibs/period.pyx

Lines 681 to 685 in c46fb76

	result = <char*>malloc(result_len * sizeof(char))
	if result is NULL:
	raise MemoryError()
	strftime(result, result_len, fmt, &c_date)

I think result here is empty but the statement of result is NULL weirdly computes to False, I don't really understand C.

[MarcoGorelli]

thanks for the ping! i spent quite some time last year on the reverse operation (strptime), but haven't looked at this code yet