Pandas Adding Row with All Values Zero

Use Setting with enlargement:

df.loc[len(df)] = 0
print (df)
   A  B  C  D  E
1  1  2  0  1  0
2  0  0  0  1 -1
3  1  1  3 -5  2
4 -3  4  2  6  0
5  2  4  1  9 -1
6  0  0  0  0  0

Or DataFrame.append with Series filled by 0 and index by columns of DataFrame:

df = df.append(pd.Series(0, index=df.columns), ignore_index=True)

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Create a new dataframe of zeroes using the shape and column list of the current. Then append:

df = pd.DataFrame([[1, 2], [3, 4],[5,6]], columns=list('AB'))
print(df)
   A  B
0  1  2
1  3  4
2  5  6

df2 = pd.DataFrame([[0]*df.shape[1]],columns=df.columns)
df = df.append(df2, ignore_index=True)
print(df)
   A  B
0  1  2
1  3  4
2  5  6
3  0  0