fix a line in PyMethodBase.cxx which make it fail to build

[ACA4DFA4]

fixed a bug in tmva/pymva/src/PyMethodBase.cxx

[phsft-bot]

Can one of the admins verify this patch?

[vepadulano]

@phsft-bot build

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac12/noimt, mac11/cxx14, windows10/cxx14
How to customize builds

[vepadulano]

Good catch @ACA4DFA4 ! I wonder why we don't see the build failure in our CI. Can you tell us your configuration and how you got the error during the build ?

[lmoneta]

I think the code before is valid C++, since you can convert const char * to std::string and you can do:

std::string s = " this  " + std::string("works ");

[vepadulano]

But the code before was doing

"Key "+key+" does not exist in the dictionary."

Which is const char * + const char * + const char *, which do not implement operator+. Or am I missing something?

godbolt example

[lmoneta]

This is correct @vepadulano. The code before compiles because we are building only with Python3 and it is compiled only in Python2 builds.
In the CI we don't have anymore Python2 only builds.

[vepadulano]

Ok I understand. I would wait for @ACA4DFA4 to tell us more about their configuration, including why do they need to build with Python2.

[phsft-bot]

Build failed on windows10/cxx14.
Running on null:C:\build\workspace\root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.root.io.stdpair.roottest_root_io_stdpair_collection_build
• projectroot.roottest.root.io.stdpair.roottest_root_io_stdpair_pair_build

[ACA4DFA4]

Ok I understand. I would wait for @ACA4DFA4 to tell us more about their configuration, including why do they need to build with Python2.

Yesterday I just found that root-v6.28/00 was released on the Releases page, then downloaded the tarball and built it with cmake (-DCMAKE_CXX_STANDARD=17), since the binary distributions don't have ROOT7 features. The OS was Ubuntu 18.04 on Windows 10 (WSL1), and there were python3.8.0, python3.7.5, python3.6.9 and python2.7.17 installed. The default python3 was python3.6.9, with numpy installed. The other 2 versions of python3 didn't have numpy or any other packages. The output of cmake configuration about python was:

-- Could NOT find Python3 (missing: Python3_INCLUDE_DIRS Python3_LIBRARIES Python3_NumPy_INCLUDE_DIRS Development NumPy Development.Module Development.Embed) (found version "3.8.0")
-- Found Python2: /usr/bin/python2.7 (found version "2.7.17") found components: Interpreter Development NumPy Development.Module Development.Embed 
...
-- Building with -fPIC
-- Found Python3: /usr/bin/python3.8 (found suitable version "3.8.0", minimum required is "3.0") found components: Interpreter 

I don't know why python3.6 was not chosen but python3.8, and why python3 was not found at the first time. In fact, I have built root-v6.26/00 with python3.6 successfully when python3.7 and python3.8 were not installed before.

[vepadulano]

@ACA4DFA4 the Python executable chosen should be whatever python3 points to, which probably will be python3.8. In the output you pasted, python3.8 is actually found, what is not found is missing: Python3_INCLUDE_DIRS Python3_LIBRARIES Python3_NumPy_INCLUDE_DIRS.... It might be enough to just install numpy with python3, but that's beside the point of this PR. In your case PyROOT will build directly with Python2 since there are some missing components for 3, hence the error you found. Thanks for the fix! This PR can be merged.