fix ImportError

[njzjz]

Fixes #1000. Add tests.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 38.82%. Comparing base (4419d45) to head (ead8f78).
Report is 224 commits behind head on devel.

Files with missing lines	Patch %	Lines
dpgen/main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #1001   +/-   ##
=======================================
  Coverage   38.82%   38.82%           
=======================================
  Files          99       99           
  Lines       17978    17977    -1     
=======================================
  Hits         6980     6980           
+ Misses      10998    10997    -1     

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