Shift NaNs to the end of their respective rows

Here’s a NumPy solution using justify –

In [455]: df
Out[455]: 
     0    1    2
0  0.0  1.0  2.0
1  NaN  1.0  2.0
2  NaN  NaN  2.0

In [456]: pd.DataFrame(justify(df.values, invalid_val=np.nan, axis=1, side="left"))
Out[456]: 
     0    1    2
0  0.0  1.0  2.0
1  1.0  2.0  NaN
2  2.0  NaN  NaN

If you want to save memory, assign it back instead –

df[:] = justify(df.values, invalid_val=np.nan, axis=1, side="left")