Python: Numpy & Pandas


A dataframe is a data structure in pandas that allows multiple datasets to be mapped to the same indices. For example, a data frame that maps dates to closing prices could be:

2000-01-09 101.01 50.89 NaN NaN
2000-01-10 100.05 50.91 NaN NaN
2015-12-31 200.89 600.25 559.50 112.37

The indices in this dataframe are the dates on the left, and the closing prices for that date are stored in each column. The ”NaN”s appear because GOOG and GLD were not publicly traded during those periods.

Reading CSVs into Dataframes

To begin using the dataframes, you need data first. Historical stock data from Yahoo is provided in the form of a CSV file, which can be easily read into a dataframe using pandas’s function read csv().

import pandas as pd

def test_run():
    df = pd.read_csv("data/AAPL.csv")
    print df

if __name__ == "__main__":

This example reads in a CSV corresponding to the historic data for AAPL (Apple, Inc) into the variable df. df is a DataFrame object, which means any DataFrame methods may be used on it.

An example of a method that can be used is max(), which returns the maximum value in the range.

import pandas as pd

def get_max_close(symbol):
    df = pd.read_csv("data/{}.csv".format(symbol))
    return df ['Close'].max()

def test_run():
    for symbol in ['AAPL','IBM']
        print "Max close "
        print symbol, get_max_close(symbol)

if __name__ == "__main__":


Matplotlib can be used to plot the data in the dataframes, as pandas can conveniently tap into the matplotlib API. Plotting data in a dataframe is as simple as calling plot() on one of the series in the frame.

Example: Plotting the Adjusted Closea price of AAPL

import pandas as pd
import matplotlib.pyplot as plt

def test_run():
	df = pd.read_csv('data/AAPL.csv')
	print df['Adj Close']

	df[['Adj Close', 'Close']].plot()

if __name__ == '__main__':

index AdjClose
0 669.79
1 660.59
2 662.74
3 680.44
4 676.27
3173 24.60
3174 24.96



There are some issues with the data that need to be solved to effectively use it in the way we want.

  • Trading days: The NYSE only trades for a certain number of days per year, which means that indexing by dates will return some results when the exchanges were not open. This poses problems for trying to pull out certain date ranges from the dataframe.
  • Multiple stocks: One of the dataframe’s powers is to be able to contain multiple ranges, which means that we need to be able to retrieve multiple datasets and store them into the dataframe.
  • Date order: The data in the Yahoo CSV are in reverse chronological order (most recent at the top), so any analysis on the dataframe will be going backwards in time, which is not ideal.

Solution to the issues

To solve the trading days problem, we’ll use an Exchange-Traded Fund (ETF) called SPY (S&P 500) to serve as a basis for what days the stock market is open. The only days that exist in the dataset for this ETF are the days the stock market traded, so if we use this as a reference and use joining on the dataframes, we can recover data on only the days which had trading.

Example: Using joins to get only traded days

start_date = '2010-01-22'
end_date = '2010-01-26'
dates = pd.date_range(start_date, end_date)
df1 = pd.DataFrame(index=dates)  # build empty dataframe

If we were to print out df1, the output would be:

Empty DataFrame
Columns : []
Index : [2010-01-22 00:00:00, 2010-01-23 00:00:00, 2010-01-25 00:00:00, 2010-01-26 00:00:00]

This empty dataframe will be the basis for the data we want to retrieve. The next step is to join this dataframe with a dataframe with the data for SPY. This will keep only indices of the SPY dataframe that also exist in the empty one.

Example: Reading in the new dataframe and joining them

dfSPY = pd.read_csv("data/SPY.csv", index_col="Date", parse_dates=True, usecols=[ 'Date','Adj Close'], na_values =['nan'])
df1 = df1.join(dfSPY)

The output would now be:

           Adj Close
2010-01-22 104.34
2010-01-23 NaN
2010-01-24 NaN
2010-01-25 104.87
2010-01-26 104.43

To get rid of the ”NaN”s, you can call dropna() on the newly joined dataframe, but there is a better way of joining them such that the ”NaN”s don’t appear in the first place. The join type is called an inner join, which joins at the intersection of the two dataframes. This way, only the dates which are in both will be kept as indices. Everything else will be thrown away.

Example: The inner join

df1 = df1.join(dfSPY, how='inner')

Multiple stocks

Reading in multiple stocks is as easy as just adding a for loop:

Example: Reading in multiple stocks into a single dataframe

dfSPY = dfSPY.rename(columns={'Adj Close': 'SPY'})
df1 = df1.join(dfSPY, how='inner')

symbols = ['GOOG', 'IBM', 'GLD']
for symbol in symbols:
    df_tmp = pd.read_csv("data/{}.csv".format(symbol),
                         usecols=['Date', 'Adj Close'], 

# rename to prevent name clash (multiple columns with same name)
df_tmp = df_tmp . rename(columns={'Adj Close': symbol})
df1 = df1.join(df_tmp, how='inner')

Here’s an example of reading and plotting multiple stocks’ closing price on one plot

Example: Reading and plotting multiple stocks

import os
import pandas as pd
import matplotlib.pyplot as plt

def plot_selected(df, columns, start_ind, end_ind):
    print df.ix[start_ind: end_ind, columns]

plot_data(df.ix[start_ind:end_ind, columns])

def symbol_to_path(symbol, base_dir="data"):
    return os.path.join(base_dir, "{}. csv". format(str(symbol)))

def get_data(symbols, dates):
    df = pd.DataFrame(index=dates)

    if 'SPY' not in symbols:
        symbols.insert(0, 'SPY')

    for symbol in symbols:
        tmp = pd.read_csv(symbol_to_path(symbol),
                          usecols=['Date', 'Close'],
        tmp = tmp.rename(columns={'Close': symbol})

        df = df.join(tmp)
        if symbol == 'SPY':
            df = df.dropna(subset=['SPY'])

    return df

def plot_data(df, title=" Stock prices"):
    ax = df.plot(title=title, fontsize=12)
    ax.set_ylabel("Closing price")

def test_run():
    dates = pd.date_range('2010-01-01', '2010-12-31')
    symbols = ['IBM ', 'GLD ']
    df = get_data(symbols, dates)
    plot_selected(df, ['SPY', 'IBM '], '2010-03-01', '2010-04-01')

if __name__ == '__main__':



Sometimes when plotting, the values of a stock will be significantly different from the other stocks such that it becomes difficult to tell some of them apart. Normalizing the data allows all of them to start at the same point and then show divergences from the initial point, making it easier to compare them at the same time.

Normalizing the dataframe is as simple as dividing the entire dataframe by its first row

Example: Normalizing a dataframe

def normalize_data(df):
    df = df / df.ix[0 ,:]
    return df



The actual data in the dataframe is actually an ndarray in NumPy (A multidimensional homogeneous array). That means we can do operations on the data using NumPy. For example, if you have a dataframe df1, the ndarray would be extracted by doing

nd1 = df1.values

Accessing a cell in the array is as simple as:

val = nd1[row, col]

You can also access subarrays by indexing with the colon:

sub = nd1[0:3, 1:3]

would capture the rectangular subarray from the first to the third rows and the second to third columns.


Note that the second part of the index is 1 past the actual index that will be the last, so 0:3 only pulls out 0, 1, and 2. Like in MATLAB, you can pull out everything using just the colon. For example:

sub = nd1[3 ,:]

would retrieve all columns of row 3.

Negative indexing

To get the last index, you can use negative numbers (the last index would be -1, second to last -2, etc.)

sub = nd1[-1, 1:3]

would get columns 1,2 of the last row

Boolean indexing/masking

Suppose we want to get the values in an array, a, which are all less than the mean. NumPy’s masking feature makes it really intuitive, as all you need to do is:

lessThanMean = a[a < a.mean()]

The array a < a.mean() would be a boolean array, which might look like

[[ True, True, False, False]]


Assigning values in an array is easy using the NumPy notation. For example, say we wanted to replace the values in the first 2x2 square of nd1 with the 2x2 square in nd2 with columns 2 and 3, and rows 3, and 4. The operation would be:

nd1[0:2, 0:2] = nd2[-2:, 2:4]

Creating an array

Creating a numpy array is as easy as passing in a normal python list into the array method:

import numpy as np
print np.array([1, 2, 3])

Creating a 2D m x n array is as simple as passing in a m-long list of n-tuples.

print np.array([(1, 2, 3), (4, 5, 6)])

would output

[[1 ,2 ,3]
 [4 ,5 ,6]]

More initializers

You can also create arrays with certain initial values.

np.empty((5, 3, 2))

initializes an ”empty” 5x3x2 dimensional array. The values in the array are actually whatever was in the memory locations of the array pointers, so the output could look like garbage.

np.ones((5, 4),

creates a 5x4 array, where the value in each cell is the integer 1.

np.random.random((5, 4))

creates a 5x4 array with random numbers from a uniform distribution in [0.0,1.0). An example result could be:

[[ 0.82897637 0.36449978 0.91209931 0.96307279]
 [ 0.63777312 0.24482194 0.5817991 0.18043012]
 [ 0.85871221 0.98874123 0.68491831 0.53831711]
 [ 0.52908238 0.81083147 0.97440602 0.81032768]
 [ 0.98566222 0.38902445 0.16922005 0.0873198 ]]

Other methods or fields, such as sum() or size() can be looked up in online documentation.