CODEWITHSUNDEEP
pythonpandasmerge
Fagui Curtain
Fagui Curtain

Reputation: 1917

Pandas merge doesn't preserve sorting order

I am trying the following:

PoliceStations_raw=pd.DataFrame(
[['BAYVIEW'   ,37.729732,-122.397981],
 ['CENTRAL'   ,37.798732,-122.409919],
 ['INGLESIDE' ,37.724676,-122.446215],
 ['MISSION'   ,37.762849,-122.422005],
 ['NORTHERN'  ,37.780186,-122.432467],
 ['PARK'      ,37.767797,-122.455287],
 ['RICHMOND'  ,37.779928,-122.464467],
 ['SOUTHERN'  ,37.772380,-122.389412],
 ['TARAVAL'   ,37.743733,-122.481500],
 ['TENDERLOIN',37.783674,-122.412899]],columns=['PdDistrict','XX','YY'])


df1=pd.DataFrame([[0,'CENTRAL'],[1,'TARAVAL'],[3,'CENTRAL'],[2,'BAYVIEW']])
df1.columns = ['Index','PdDistrict']


  Index PdDistrict
0   0   CENTRAL
1   1   TARAVAL
2   3   CENTRAL
3   2   BAYVIEW

despite having typed in sort=False, the returned object has merged the table but is using PdDistrict as some Index and has changed the order of the rows of the original left dataframe.

pd.merge(df1,PoliceStations_raw,sort=False)

returns this (note that the order of PdDistrict has changed)

  Index PdDistrict  XX        YY
0   0   CENTRAL 37.798732   -122.409919
1   3   CENTRAL 37.798732   -122.409919
2   1   TARAVAL 37.743733   -122.481500
3   2   BAYVIEW 37.729732   -122.397981

Upvotes: 1

Views: 3843

Answers (1)

hichris123
hichris123

Reputation: 10223

You need to specify how you want the two dataframes merged. By default, an inner join is emulated by merge(). However, by specifying you want a left join, the sort order from df1 is preserved. Thus, you simply need to add how='left':

>>> pd.merge(df1, PoliceStations_raw, how='left')
   Index PdDistrict         XX          YY
0      0    CENTRAL  37.798732 -122.409919
1      1    TARAVAL  37.743733 -122.481500
2      3    CENTRAL  37.798732 -122.409919
3      2    BAYVIEW  37.729732 -122.397981

Additionally, sort=False is the default behavior -- you don't need to specify that.

Upvotes: 5

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