"""This file contains code for use with "Think Stats",
by Allen B. Downey, available from greenteapress.com
Copyright 2010 Allen B. Downey
License: GNU GPLv3 http://www.gnu.org/licenses/gpl.html
"""
import sys
import gzip
import os
class Record(object):
"""Represents a record."""
class Respondent(Record):
"""Represents a respondent."""
class Pregnancy(Record):
"""Represents a pregnancy."""
class Table(object):
"""Represents a table as a list of objects"""
def __init__(self):
self.records = []
def __len__(self):
return len(self.records)
def ReadFile(self, data_dir, filename, fields, constructor, n=None):
"""Reads a compressed data file builds one object per record.
Args:
data_dir: string directory name
filename: string name of the file to read
fields: sequence of (name, start, end, case) tuples specifying
the fields to extract
constructor: what kind of object to create
"""
filename = os.path.join(data_dir, filename)
if filename.endswith('gz'):
fp = gzip.open(filename)
else:
fp = open(filename)
for i, line in enumerate(fp):
if i == n:
break
record = self.MakeRecord(line, fields, constructor)
self.AddRecord(record)
fp.close()
def MakeRecord(self, line, fields, constructor):
"""Scans a line and returns an object with the appropriate fields.
Args:
line: string line from a data file
fields: sequence of (name, start, end, cast) tuples specifying
the fields to extract
constructor: callable that makes an object for the record.
Returns:
Record with appropriate fields.
"""
obj = constructor()
for (field, start, end, cast) in fields:
try:
s = line[start-1:end]
val = cast(s)
except ValueError:
val = 'NA'
setattr(obj, field, val)
return obj
def AddRecord(self, record):
"""Adds a record to this table.
Args:
record: an object of one of the record types.
"""
self.records.append(record)
def ExtendRecords(self, records):
"""Adds records to this table.
Args:
records: a sequence of record object
"""
self.records.extend(records)
def Recode(self):
"""Child classes can override this to recode values."""
pass
class Respondents(Table):
"""Represents the respondent table."""
def ReadRecords(self, data_dir='.', n=None):
filename = self.GetFilename()
self.ReadFile(data_dir, filename, self.GetFields(), Respondent, n)
self.Recode()
def GetFilename(self):
return '2002FemResp.dat.gz'
def GetFields(self):
"""Returns a tuple specifying the fields to extract.
The elements of the tuple are field, start, end, case.
field is the name of the variable
start and end are the indices as specified in the NSFG docs
cast is a callable that converts the result to int, float, etc.
"""
return [
('caseid', 1, 12, int),
]
class Pregnancies(Table):
"""Contains survey data about a Pregnancy."""
def ReadRecords(self, data_dir='.', n=None):
filename = self.GetFilename()
self.ReadFile(data_dir, filename, self.GetFields(), Pregnancy, n)
self.Recode()
def GetFilename(self):
return '2002FemPreg.dat.gz'
def GetFields(self):
"""Gets information about the fields to extract from the survey data.
Documentation of the fields for Cycle 6 is at
http://nsfg.icpsr.umich.edu/cocoon/WebDocs/NSFG/public/index.htm
Returns:
sequence of (name, start, end, type) tuples
"""
return [
('caseid', 1, 12, int),
('nbrnaliv', 22, 22, int),
('babysex', 56, 56, int),
('birthwgt_lb', 57, 58, int),
('birthwgt_oz', 59, 60, int),
('prglength', 275, 276, int),
('outcome', 277, 277, int),
('birthord', 278, 279, int),
('agepreg', 284, 287, int),
('finalwgt', 423, 440, float),
]
def Recode(self):
for rec in self.records:
try:
if rec.agepreg != 'NA':
rec.agepreg /= 100.0
except AttributeError:
pass
try:
if (rec.birthwgt_lb != 'NA' and rec.birthwgt_lb < 20 and
rec.birthwgt_oz != 'NA' and rec.birthwgt_oz <= 16):
rec.totalwgt_oz = rec.birthwgt_lb * 16 + rec.birthwgt_oz
else:
rec.totalwgt_oz = 'NA'
except AttributeError:
pass
def main(name, data_dir='.'):
resp = Respondents()
resp.ReadRecords(data_dir)
print 'Number of respondents', len(resp.records)
preg = Pregnancies()
preg.ReadRecords(data_dir)
print 'Number of pregnancies', len(preg.records)
if __name__ == '__main__':
main(*sys.argv)