Memory usage increase continuously about weld HOT 4 OPEN

#!/usr/bin/python

# The usual preamble
import pandas as pd
import grizzly.grizzly as gr
import time

# Get data (NYC 311 service request dataset) and start cleanup
na_values = ['NO CLUE', 'N/A', '0']
raw_requests = pd.read_csv('data/311-service-requests.csv',
                           na_values=na_values, dtype={'Incident Zip': str})
print "Done reading input file..."

for i in range(500):
    requests = gr.DataFrameWeld(raw_requests)
    start = time.time()

    # Fix requests with extra digits
    requests['Incident Zip'] = requests['Incident Zip'].str.slice(0, 5)

    # Fix requests with 00000 zipcodes
    zero_zips = requests['Incident Zip'] == '00000'
    requests['Incident Zip'][zero_zips] = "nan"

    # Display unique incident zips again (this time cleaned)
    print requests['Incident Zip'].unique().evaluate()
    end = time.time()

    print "Total end-to-end time, including compilation: %.2f" % (end - start)