How to Use Excel's COUPNUM Function in Pandas

In finance, the COUPNUM function is crucial for understanding the cash flow from fixed-income securities, like bonds. It determines the number of coupon payments to be received between the purchase and maturity dates.

This page demonstrates how to translate the COUPNUM function from Excel to Python using pandas, making it accessible for analysts familiar with Excel but new to Python.

Implementing the Coupon Number function in Pandas#

To replicate the COUPNUM function in Python using pandas, one must understand date manipulation and financial concepts. Here's how to calculate the number of coupon payments between two dates in pandas.

Calculating Coupon Count Between Dates#

To calculate the number of coupon payments between two dates, you first define the settlement and maturity dates. Then, based on the coupon frequency, you create a date range and count the number of dates falling within this range.

In Excel, you would simply use =COUPNUM(). In pandas, you'll need to use a combination of date functions and custom calculations.

In the code below, we define a function called get_count_coupons_payable() that takes three arguments: the settlement date, the maturity date, and the coupon frequency. The function returns the number of coupon payments between the settlement and maturity dates.

from datetime import datetime
from dateutil.relativedelta import relativedelta

def get_count_coupons_payable(settlement, maturity, freq):
    # Calculate the period of coupon payments in months
    period = 12 // freq
    coupon_date = settlement_date
    coupon_count = 0

    # Loop until the coupon date is after the maturity date
    while coupon_date < maturity_date:
        coupon_date += relativedelta(months=period)
        if coupon_date <= maturity_date:
            coupon_count += 1

    return coupon_count

# Convert string dates to datetime objects
settlement_date = pd.to_datetime("1/15/23")
maturity_date = pd.to_datetime("1/15/28")

# Calculate the number of coupon payments
num_coupon_payments = get_count_coupons_payable(settlement_date, maturity_date, 1)

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Calculating Coupon Count Between Dates in pandas Dataframe#

If instead of calculating the number of coupon payments paid out by just one security, you want to calculate the number of coupon payments paid out by multiple securities, you can use the apply() function to apply the count_coupons_payable() function to each row of the dataframe.

The code looks like this:

# Create a dataframe with the settlement and maturity dates
df = pd.DataFrame({
    'settlement': ['1/15/23', '1/15/23', '1/15/23'],
    'maturity': ['1/15/28', '1/15/28', '1/15/28'],
    'freq': [1, 2, 4]
})

# Convert settlement and maturity columns to datetimes
df['settlement'] = pd.to_datetime(df['settlement'])
df['maturity'] = pd.to_datetime(df['maturity'])

# Apply the get_count_coupons_payable() function to each row of the dataframe
df['num_coupon_payments'] = df.apply(lambda x: get_count_coupons_payable(x['settlement'], x['maturity'], x['freq']), axis=1)

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Calculating Remaining Coupon Payments#

If instead of calculating the number of coupon payments between two dates, you want to calculate the number of remaining coupon payments, you can use today's date as the settlement date and the maturity date as the maturity date.

from datetime import datetime

# Get input dates
today = datetime.today()
maturity = pd.to_datetime("1/15/28")

# Calculate the number of remaining coupon payments
num_coupon_payments = get_count_coupons_payable(today, maturity, 1)

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Common mistakes when using COUPNUM in Python#

While implementing COUPNUM in pandas, it's crucial ensure that you're using the correct data types. Unlike Excel, pandas cannot interpret dates that are not in a recognized format.

Incorrect Data Types#

A common mistake is using strings for dates. Ensure that the dates are in a recognized format, such as 'YYYY-MM-DD' before using them to calculate the number of coupon payments.

# Cast string to date
settlement_date = pd.to_datetime("1/15/23")

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Understanding the Coupon Number Formula in Excel#

The COUPNUM function in Excel calculates the number of coupons (interest payments) due between the settlement and maturity dates of a bond.

=COUPNUM(settlement, maturity, frequency, [basis])

COUPNUM Excel Syntax

Examples