python - How to append to DataFrame inside function and return changes outside the function

Question

I have a python script that creates a DataFrame df, conencts to an API, and appends data to the DataFrame in a function. However the dataframe is empty outside of the function.

The function is "get_stock_price(client, ticker, option_dates, df)" I've shortened the code to show the part where I try appending to the df.

#rewrite with one reqMktData and tickType only

from datetime import datetime
from threading import Thread
import time
from ibapi.client import EClient, Contract
from ibapi.wrapper import EWrapper
from ibapi.utils import iswrapper
import math
import pandas as pd
import numpy as np

df = pd.DataFrame([], columns=['Underlying', 'Expiry', 'Strike', 'Right'])

class TestApp(EWrapper,EClient):

    def __init__(self, addr, port, client_id):
        EWrapper.__init__(self)
        EClient. __init__(self, self)

        # Initialize variables
        # Connect to TWS API

#--functions that connect to API to get data---


def get_stock_price(client, ticker, option_dates, df):

#set contract and call API functions

    #get strikes between q_up and q_down, append to df? 
    for x in range(len(option_dates)):
        for strike in client.strikes:
            if strike > q_down[x] and strike < q_up[x]:
                df=df.append({
                    'Underlying': ticker,
                    'Expiry': option_dates[x],
                    'Strike': strike,
                    'Strike': strike,
                    'Right' : 'C',
                    }, ignore_index=True)
                return df

    df=df.reindex(df.index.repeat(2))
    df=df.reset_index(drop=True)

    print(df)

    df['Underlying']='test'
    return(df)


def main():
    client = TestApp('127.0.0.1', 7497, 126)

    ticker = 'AMD'
    option_dates = ['20200821', '20200918', '20201016']
    get_stock_price(client, ticker, option_dates, df)

    client.done=True
    client.disconnect()
    print('try df again')
    print(df)


if __name__ == "__main__":
    main()

From reading answers on another question I tried adding

return df

After the append, also tried:

return df.append({
                    'Underlying': ticker,
                    'Expiry': option_dates[x],
                    'Strike': strike,
                    'Strike': strike,
                    'Right' : 'C',
                    }, ignore_index=True)

and as a test:

df['Underlying']='test'
    return(df)

None of these seemed to work, when I print(df) at the end of the main() function it's empty.

Here's the output:

...
288        AMD  20201016   108.0     P
289        AMD  20201016   108.0     C
290        AMD  20201016   109.0     P
291        AMD  20201016   109.0     C

[292 rows x 4 columns]
try printing df again
Empty DataFrame
Columns: [Underlying, Expiry, Strike, Right]
Index: []
...