title: Pandas left join gives NaN
body: <h3>DataFrameA</h3>
  
  <pre><code>                                 RunId  isClean  isFinished
  0                 APAC_P1_HSFR_REGTEST      0.0         1.0
  0                      APAC_P1_REGTEST      1.0         1.0
  0                APAC_P2a_HSFR_REGTEST      0.0         0.0
  0                     APAC_P2a_REGTEST      0.0         1.0
  0                APAC_P2b_HSFR_REGTEST      0.0         0.0
  0                     APAC_P2b_REGTEST      1.0         1.0
  0                APAC_P2c_HSFR_REGTEST      0.0         0.0
  0                     APAC_P2c_REGTEST      0.0         1.0
  0                APAC_P3a_HSFR_REGTEST      0.0         0.0
  0                     APAC_P3a_REGTEST      0.0         0.0
  0                APAC_P3b_HSFR_REGTEST      0.0         0.0
  0                     APAC_P3b_REGTEST      0.0         0.0
  0                      Cliquet_REGTEST      0.0         1.0
  </code></pre>
  
  <h3>DataFrameB</h3>
  
  <pre><code>                             RunId  Status
  0            APAC_P2a_HSFR_REGTEST   Error
  2            APAC_P2b_HSFR_REGTEST   Error
  4            APAC_P2c_HSFR_REGTEST   Error
  6            APAC_P3a_HSFR_REGTEST   Error
  8            APAC_P3b_HSFR_REGTEST   Error
  10             CS_P10_HSFR_REGTEST   Error
  12             CS_P11_HSFR_REGTEST   Error
  14              CS_P1_HSFR_REGTEST   Error
  16              CS_P2_HSFR_REGTEST   Error
  18              CS_P3_HSFR_REGTEST   Error
  20              CS_P4_HSFR_REGTEST   Error
  22              CS_P5_HSFR_REGTEST   Error
  24              CS_P6_HSFR_REGTEST   Error
  </code></pre>
  
  <h3>DataFrameA.join(DataFrameB,on='RunId',how='left',rsuffix='_y')</h3>
  
  <pre><code>                                 RunId  isClean  isFinished RunId_y Status
  0                 APAC_P1_HSFR_REGTEST      0.0         1.0     NaN    NaN
  0                      APAC_P1_REGTEST      1.0         1.0     NaN    NaN
  0                APAC_P2a_HSFR_REGTEST      0.0         0.0     NaN    NaN
  0                     APAC_P2a_REGTEST      0.0         1.0     NaN    NaN
  0                APAC_P2b_HSFR_REGTEST      0.0         0.0     NaN    NaN
  0                     APAC_P2b_REGTEST      1.0         1.0     NaN    NaN
  0                APAC_P2c_HSFR_REGTEST      0.0         0.0     NaN    NaN
  0                     APAC_P2c_REGTEST      0.0         1.0     NaN    NaN
  0                APAC_P3a_HSFR_REGTEST      0.0         0.0     NaN    NaN
  0                     APAC_P3a_REGTEST      0.0         0.0     NaN    NaN
  0                APAC_P3b_HSFR_REGTEST      0.0         0.0     NaN    NaN
  0                     APAC_P3b_REGTEST      0.0         0.0     NaN    NaN
  0                      Cliquet_REGTEST      0.0         1.0     NaN    NaN
  </code></pre>
  
  <p>A question, why all <code>NaNs</code> ? There are nonNaN intersections like <code>APAC_P2a_HSFR_REGTEST</code>. Sorry if it's a dumb question, but googling did not helped.</p>