Conditions True and false and I get None value - pandas

Question

This is my data
!pip install yfinance
import yfinance as yf
from pandas_datareader import data
import matplotlib.pyplot as plt
import pandas as pd
import datetime as dt
import urllib.request, json
import os
import numpy as np
data=yf.download('AAPL', period='max', interval='5d' )
# find the log return which is equal to log(1+ri)
data['LogReturn'] = np.log(data['Close']).diff()
data['LogReturn'] = data['LogReturn'].shift(-1)
Fast=10
Slow=30
data['SlowSMA'] = data['Close'].rolling(Slow).mean()
data['FastSMA'] = data['Close'].rolling(Fast).mean()
data['Signal']=np.where(data['FastSMA'] >= data['SlowSMA'], 1, 0)
data['PrevSignal']=data['Signal'].shift(1)
data['Buy'] = (data['PrevSignal'] == 0) & (data['Signal'] == 1)
data['Sell'] = (data['PrevSignal'] == 1) & (data['Signal'] == 0)
def assign_is_invested(row): ## we will look at each row
global is_invested # we can change it outside the function
if is_invested and row['Sell']:
is_invested=False
if not is_invested and row['Buy']:
is_invested=True
return is_invested
data['IsInvested'] = data.apply(assign_is_invested, axis=1)
When I run the above function to get the IsInvested column None and I am expecting True or False. Why is that?

Answer 1

I believe that the problem is with the indentation of the return.
is_invested= False # we are not invested yet no money
def assign_is_invested(row): ## we will look at each row
global is_invested # we can change it outside the function
if is_invested and row['Sell']:
is_invested=False
if not is_invested and row['Buy']:
is_invested=True
return is_invested
data['IsInvested'] = data.apply(assign_is_invested, axis=1)
The function was only returning the is_invested variable if the data matched the criteria in the second IF statement.