import pandas as pd

df1 = pd.DataFrame({"mes":["enero","febrero","marzo", "mayo"], "ventas":[14, 8, 12, 17]})
print df1
print "\n\n"

df2 = pd.DataFrame({"mes":["febrero","enero","marzo", "abril"], "costos":[7, 6, 8, 5]})
print df2
print "\n\n"

df3 = pd.merge(df1, df2)
print df3
print "\n\n"

df4 = pd.merge(df1, df2, how = "outer")
print df4
print "\n\n"