# concatenate
pd.concat([df1, df2], ignore_index=False, key=['str1', 'str2', 'str3'], sort=True)
# use only "key" if index is ignored

# exclude columns if different
join='inner'

# validating / check for multiple index columns
verify_integrity=True

# append (= easy version of concatenate)
# here, join always = 'outer'
df1.append([df2, df3], ignore_index=True, sort=True)