pip import issue

[bensworth]

I tried to install PyAMG in a conda environment using pip, but cannot import:

>>> import pyamg
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/__init__.py", line 12, in <module>
    from .classical import ruge_stuben_solver
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/classical/__init__.py", line 4, in <module>
    from .classical import *
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/classical/classical.py", line 9, in <module>
    from pyamg.relaxation.smoothing import change_smoothers
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/relaxation/smoothing.py", line 5, in <module>
    from . import relaxation
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/relaxation/relaxation.py", line 10, in <module>
    from pyamg.util.utils import type_prep, get_diagonal, get_block_diag
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/util/__init__.py", line 7, in <module>
    from .utils import *
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/util/utils.py", line 13, in <module>
    import pyamg.amg_core
  File "/opt/anaconda3/lib/python3.8/site-packages/pyamg/amg_core/__init__.py", line 5, in <module>
    from .evolution_strength import *
ImportError: dlopen(/opt/anaconda3/lib/python3.8/site-packages/pyamg/amg_core/evolution_strength.cpython-38-darwin.so, 2): Symbol not found: __ZNKSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE4findEPKcmm
  Referenced from: /opt/anaconda3/lib/python3.8/site-packages/pyamg/amg_core/evolution_strength.cpython-38-darwin.so
  Expected in: /usr/lib/libstdc++.6.dylib
 in /opt/anaconda3/lib/python3.8/site-packages/pyamg/amg_core/evolution_strength.cpython-38-darwin.so

Forced a reinstall/upgrade too via pip install --upgrade --force-reinstall pyamg but the problem persisted

[bensworth]

Oops, my force reinstall was using the wrong pip. Forcing a reinstall fixed the issue.

[lukeolson]

Sounds good -- let me know if you run into problems again. Often Symbol not found: __ZNKSt7_... types of things indicates a problem with the wrong compiler or incorrect Python version.