Fix escrow leak; #1673

masspay.py

@@ -25,7 +25,7 @@
 import getpass
 import os
 import sys
-from decimal import Decimal as D, ROUND_UP
+from decimal import Decimal as D
 
 import requests
 
@@ -40,9 +40,6 @@
 def round_(d):
  return d.quantize(D('0.01'))
 
-def round_up(d):
- return d.quantize(D('0.01'), ROUND_UP)
-
 def print_rule():
  print("-" * 80)
 
@@ -61,9 +58,41 @@ def __init__(self, rec):
  self.fee = D(0)
  self.net = self.gross
 
- def assess_fee(self, fee):
+ def assess_fee(self):
+ fee = self.gross - round_(self.gross / D('1.02')) # 2% fee
  self.fee += fee
  self.net -= fee
+ if self.net % 1 in (D('0.25'), D('0.75')):
+
+ # Prevent an escrow leak. It's complicated, but it goes something
+ # like this:
+ #
+ # 1. We want to pass PayPal's fees through to each payee.
+ #
+ # 2. There is no option to have the receiver pay the fee, as
+ # there is with Instant Transfer.
+ #
+ # 3. We have to subtract the fee before uploading the spreadsheet
+ # to PayPal.
+ #
+ # 4. If we upload 15.24, PayPal upcharges to 15.54.
+ #
+ # 6. If we upload 15.25, PayPal upcharges to 15.56.
+ #
+ # 7. They only accept whole cents. We can't upload 15.245.
+ #
+ # 8. What if we want to hit 15.55?
+ #
+ # 9. We can't.
+ #
+ # 10. Our solution is to leave a penny behind in Gittip for
+ # affected payees.
+ #
+ # See also: https://github.com/gittip/www.gittip.com/issues/1673.
+
+ self.gross -= D('0.01')
+ self.net -= D('0.01')
+ return fee
 
 
 def compute_input_csv():
@@ -105,42 +134,10 @@ def compute_output_csvs():
  payees = [Payee(rec) for rec in csv.reader(open(INPUT_CSV))]
  payees.sort(key=lambda o: o.gross, reverse=True)
 
- total_gross = sum([p.gross for p in payees])
- total_fees = total_gross - round_(total_gross / D('1.02')) # 2% fee
- total_net = D('0.00')
-
- for payee in payees:
- payee.gross_perc = payee.gross / total_gross
- payee.assess_fee(round_(total_fees * payee.gross_perc))
-
- fee_check = sum([p.fee for p in payees])
- if fee_check != total_fees:
-
- # Up to one penny per payee is okay.
- fee_difference = total_fees - fee_check
- fee_tolerance = D('0.0{}'.format(len((payees)))) # one penny per payee
- print_rule()
- print()
- print("Fee rounding error tolerance: {}".format(fee_tolerance))
- print("Accumulated fee rounding error: {}".format(fee_difference))
- print()
- assert fee_difference < fee_tolerance
-
- # Distribute rounding errors.
- for payee in reversed(payees):
- allotment = round_up(fee_difference * payee.gross_perc)
- payee.assess_fee(allotment)
- print(" {} => {}".format(allotment, payee.email))
-
- fee_difference -= allotment
- if fee_difference == 0:
- break
- print()
-
+ total_fees = sum([payee.assess_fee() for payee in payees]) # side-effective!
  total_net = sum([p.net for p in payees])
- fee_check = sum([p.fee for p in payees])
- assert fee_check == total_fees
- assert total_net + total_fees == total_gross
+ total_gross = sum([p.gross for p in payees])
+ assert total_fees + total_net == total_gross
 
  paypal_csv = csv.writer(open(PAYPAL_CSV, 'w+'))
  gittip_csv = csv.writer(open(GITTIP_CSV, 'w+'))