Daily Returns

Daily Returns

Python code to implement daily returns

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

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

def get_data(symbols, dates):
    """Read stock data (adjusted close) for given symbols from CSV files."""
    df = pd.DataFrame(index=dates)
    if 'SPY' not in symbols:  # add SPY for reference, if absent
        symbols.insert(0, 'SPY')

    for symbol in symbols:
        df_temp = pd.read_csv(symbol_to_path(symbol), index_col='Date',
                              parse_dates=True, usecols=['Date', 'Adj Close'], na_values=['nan'])
        df_temp = df_temp.rename(columns={'Adj Close': symbol})
        df = df.join(df_temp)
        if symbol == 'SPY':  # drop dates SPY did not trade
            df = df.dropna(subset=["SPY"])

    return df

def plot_data(df, title="Stock prices", xlabel="Date", ylabel="Price"):
    """Plot stock prices with a custom title and meaningful axis labels."""
    ax = df.plot(title=title, fontsize=12)
    ax.set_xlabel(xlabel)
    ax.set_ylabel(ylabel)
    plt.show()

def compute_daily_returns(df):
    """Compute and return the daily return values."""
    daily_returns = df.pct_change()
    # Daily return values for the first date cannot be calculated. Set these to zero.
    daily_returns.ix[0, :] = 0

    # Alternative method
    # daily_returns = (df / df.shift(1)) - 1
    # daily_returns.ix[0, :] = 0

    # Another alternative method
    # daily_returns = df.copy()
    # compute daily returns for row 1 onwards
    # daily_returns[1:] = (daily_returns[1:] / daily_returns[-1:].values) - 1
    # daily_returns.ix[0, :] = 0 # set daily returns for row 0 to 0

    return daily_returns

def test_run():
    # Read data
    dates = pd.date_range('2012-07-01', '2012-07-31')  # one month only
    symbols = ['SPY', 'XOM']
    df = get_data(symbols, dates)
    plot_data(df)

    # Compute daily returns
    daily_returns = compute_daily_returns(df)
    plot_data(daily_returns, title="Daily returns", ylabel="Daily returns")

if __name__ == "__main__":
    test_run()

Here are the two charts.

Observe that SPY and XOM stocks are somehow connected. It seems that SPY is following XOM stock changes.