Improve Row Append Performance On Pandas DataFrames

I think the best way to do it is, if you know the data you are going to receive, allocate before hand.

import numpy as np
import pandas as pd

random_matrix = np.random.randn(100, 100)
insert_df = pd.DataFrame(random_matrix)

df = pd.DataFrame(columns=range(100), index=range(200))
df.loc[range(100), df.columns] = random_matrix
df.loc[range(100, 200), df.columns] = random_matrix

This is the pattern that I think makes the most sense. append will be faster if you have a very small dataframe, but it doesn't scale.

In [1]: import numpy as np; import pandas as pd

In [2]: random_matrix = np.random.randn(100, 100)
   ...: insert_df = pd.DataFrame(random_matrix)
   ...: df = pd.DataFrame(np.random.randn(100, 100))

In [2]: %timeit df.append(insert_df)
272 µs ± 2.36 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [3]: %timeit df.loc[range(100), df.columns] = random_matrix
493 µs ± 4.25 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [4]: %timeit df.loc[range(100), df.columns] = insert_df
821 µs ± 8.68 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

When we run this with a 100,000 row dataframe, we see much more dramatic results.

In [1]: df = pd.DataFrame(np.random.randn(100_000, 100))

In [2]: %timeit df.append(insert_df)
17.9 ms ± 253 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

In [3]: %timeit df.loc[range(100), df.columns] = random_matrix
465 µs ± 13.7 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [4]: %timeit df.loc[range(99_900, 100_000), df.columns] = random_matrix
465 µs ± 5.75 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [5]: %timeit df.loc[range(99_900, 100_000), df.columns] = insert_df
1.02 ms ± 3.42 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

So we can see an append is about 17 times slower than an insert with a dataframe, and 35 times slower than an insert with a numpy array.

Appending rows to lists is far more efficient than to a DataFrame. Hence you would want to

1. append the rows to a list.
2. Then convert it into DataFrame and
3. set the index as required.

I also used the dataframe's append function inside a loop and I was perplexed how slow it ran.

A useful example for those who are suffering, based on the correct answer on this page.

Python version: 3

Pandas version: 0.20.3

# the dictionary to pass to pandas dataframe
d = {}

# a counter to use to add entries to "dict"
i = 0 

# Example data to loop and append to a dataframe
data = [{"foo": "foo_val_1", "bar": "bar_val_1"}, 
       {"foo": "foo_val_2", "bar": "bar_val_2"}]

# the loop
for entry in data:

    # add a dictionary entry to the final dictionary
    d[i] = {"col_1_title": entry['foo'], "col_2_title": entry['bar']}
    
    # increment the counter
    i = i + 1

# create the dataframe using 'from_dict'
# important to set the 'orient' parameter to "index" to make the keys as rows
df = DataFrame.from_dict(d, "index")

The "from_dict" function: https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.from_dict.html