1, df2, fill_method="ffill", left_by="group")
      key  lvalue group  rvalue
    0   a       1     a     NaN
    1   b       1     a     1.0
    2   c       2     a     2.0
    3   d       2     a     3.0
    4   e       3     a     3.0
    5   a       1     b     NaN
    6   b       1     b     1.0
    7   c       2     b     2.0
    8   d       2     b     3.0
    9   e       3     b     3.0
    r[