mpath special cases for the sage backend and the mpf class is missing. Why?

[Rocky Bernstein]

Even using mpmath 1.2.1 when inside sage. The mpf class is not defined. 

For example: 

sage: import mpmath
sage: mpmath.ctx_mp_python.mpf
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-8e8eed115623> in <module>
----> 1 mpmath.ctx_mp_python.mpf

AttributeError: module 'mpmath.ctx_mp_python' has no attribute 'mpf'

And if you look at mpmath/libmp/libmpf.py you will see lines like: 

 if BACKEND == 'sage': 

Some of this I guess has to do with differences in Python pickle between Pythoin 2 and Python3. But if that is so, then wouldn't a test on the Python version be more direct? 

And why is the mpf class missing in this case? 

See the thread in https://github.com/mathics/Mathics/issues/1169#issuecomment-806532433 for the full details.

[Isuru Fernando]

You shouldn't be using `mpmath.ctx_mp_python.mpf`. Just use `mpmath.mpf`.

Isuru

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[Rocky Bernstein]

I think this fixes the problem we see. Thanks!

Aside from asking from my asking here, was this noted somewhere else?

[Isuru Fernando]

> Aside from asking from my asking here, was this noted somewhere else?

A good rule is not to use any function/class/module that is not documented. While `mpmath.mpf` is documented, `mpmath.ctx_mp_python` module is not.

Isuru

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