import pandas as pd

df1 = pd.DataFrame([[1, 2], [3, 4]], columns=list('AB'))
df2 = pd.DataFrame([[5, 6], [7, 8]], columns=list('AB'))

mydf1 = pd.concat([df1, df2])
#mydf1 = df1.append(df2)
print('mydf1:\n', mydf1)

df1 = pd.DataFrame([[1, 2], [3, 4]], columns=list('BA'))
mydf2 = pd.concat([df1,df2], ignore_index=True) 
print('mydf2:\n', mydf2)

mydf3 = pd.concat([df1,df2], ignore_index=True, sort=True) 
print('mydf3:\n', mydf3)

mydf4 = pd.concat([df1,df2], sort=True) 
print('mydf4:\n', mydf4)