In Python's tutorial one can read that Python's original implementation is in C;
On the other hand, the Python implementation, written in C, (...)
I'm very curious why was Python written in C and not C++?
I'd like to know the reasoning behind this decision and the answer should be supported by historical references (and not opinion based).
A better question might be: "Why isn't Python written in Python?"
More to the point, once enough primitives for Python classes and objects are written in C, those can be used for writing the rest of the interpreter, so you wouldn't gain anything by using C++ instead.
From everything I've seen, it's a combination of practical and historical reasons. The (mostly) historical reason is that CPython 1.0 was released in 1989. At that time, C was just recently standardized. C++ was almost unknown and decidedly non-portable, because almost nobody had a C++ compiler.
Although C++ is much more widespread and easily available today, it would still take a fair amount of work to rewrite CPython into the subset of C that's compatible with C++. By itself, that work would provide little or no real benefit.
It's a bit like Joel's blog post about starting over and doing a complete rewrite being the worst mistake a software company can make. I'd counter that by pointing to Microsoft's conversion from the Windows 3.0 core to the Windows NT core, and Apple's conversion from MacOS 9 to Mac OS/X. Neither one killed the company -- but both were definitely large, expensive, long-term projects. Both also point to something that's crucial to success: maintaining both code bases for long enough that (most) users can switch to the new code base at their leisure, based on (at least perceived) benefits.
For a development team the size of Python's, however, that kind of change is much more difficult. Even the change from Python 2 to 3 has taken quite a bit of work, and required a similar overlap. At least in that case, however, there are direct benefits to the changes, which rewriting into C++ (by itself) wouldn't (at least immediately) provide.
Linus Torvalds's rant against C++ was brought up, so I'll mention that as well. Nothing I've seen from Guido indicates that he has that sort of strong, negative feelings toward C++. About the worst I've seen him say is that teaching C++ is often a disaster -- but he immediately went on to say that this is largely because the teachers didn't/don't know C++.
I also think that while it's possible to convert a lot of C code to C++ with relative ease, that getting much real advantage from C++ requires not only quite a bit more rewriting than that, but also requires substantial re-education of most developers involved. Most well-written C++ is substantially different from well-written C to do the same things. It's not just a matter of changing
cout, by any stretch of the imagination.
I think the reason why it was originally written in ANSI C89 is quite simply because back then, C++ was just not a workable choice what with incompatibilities between different compilers and such. I mean, it took until, what was it, 2005, to come up with an ABI specification that would allow code compiled with one compiler to call code compiled with a different compiler?
The more interesting question is why it is still written in C89.
And there is a surprising answer: because people actually use Python on platforms for which no C++ and no C99 compiler exists! When the Forth-inspired threaded-code interpreter optimizations were merged, there was a huge discussion about it, because the code (necessarily) used computed
goto which is not a part of C89. There were apparently real fears that this feature might not be available on some of the platforms that Python is currently used on.
The same thing happened with Unladen Swallow, wich uses LLVM, which is written in C++. It was made very clear that a requirement for merging Unladen Swallow into CPython would be that you can compile it without the JIT compiler, since there are platforms people run Python on, for which no C++ compiler exists.
Of course, nowadays, CPython is no longer the only Python implementation. There is PyPy, which is written in RPython (a statically typed subset of Python), Jython in Java, IronPython in C#, Pynie in NQP and PIR and so on.