Lambda behaves differently when invoked from Console (test) and through AWS Gateway REST API - python-3.x

Question

Problem: Pandas behaves differently when
I call it from the "invoke" test (and via AWS Gateway which is pointed to the said Lambda)
I call it through a REST API via AWS Gateway
Lambda works fine with A) (it returns intended values) but does not work with B) (returns NaN values).
Details: I have a simple python script in a Lambda (python 3.8). With the code, I have uploaded pandas, numpy and a .csv file into the Lambda. I built a rest API with AWS Gateway that uses the Lambda as a resource.
My objective is to load the .csv into a Pandas dataframe when Lambda is requested, then filter the dataframe with the event parameters to find particular values, and return those values.
Sample code below.
Any help would be appreciated.
Code
Lambda:
def lambda_handler(event, context):
# extract filter values from request
vendor = event['Vendor']
# load the csv
data = pd.read_csv('vendor_data.csv')
print("Data shape", data.shape)
# correct types
data.quantity = (data.quantity).apply(lambda x: int(x))
data.loc[:,'Vendor'] = data['Vendor'].astype('str') # I also tried astype('string'), doesn't change
# filter the dataframe with Vendor == vendor
condition = (data.Vendor.str.contains(str(vendor))) #I also tried (data.Vendor == vendor)
data = data.loc[condition]
#verify it worked: it works with 'invoke' but not through rest API
print('First five rows', data[['Vendor']].head())
data = data['quantity'].apply(lambda x: int(x))
sum = data.sum()
return sum
Gateway Rest API
Request model template:
{ "$schema": "http://json-schema.org/draft-04/schema#",
"title": "Request",
"type": "object",
"properties": {
"Vendor": {
"type": "string"
},
},
"required": ["Vendor"]
}
Response model template:
{
"$schema": "http://json-schema.org/draft-04/schema#",
"title": "Response",
"type": "object",
"properties": {
"sum": {
"type": "number"
}
}
}
Example test (post to rest API)
PARAMS = {
"Vendor": "2202630"
}
PARAMS = json.dumps(PARAMS)
print(PARAMS)
r = requests.post(url = URL, data = PARAMS)
print(r.content)
results in:
{ "Vendor": "2202630"}
b'{"statusCode": 200, "body": NaN, "event": {"Vendor": "2202630"}, "data": "[]"}'
Example test (through AWS Gateway):
Request:
{
"Vendor": "2202630"
}
Response body:
{
"statusCode": 200,
"body": 599,
"event": {
"Vendor": "22026305"
},
"data": "[50 44 38 45 50 45 38 26 18 31 21 9 53 47 45 39]"
}