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Given the following text description, write Python code to implement the functionality described below step by step
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Description:
 rvlib Change Log Statistics
This notebook generates the summary statistics for a package. 
It assumes you are running this under the .ci directory at the toplevel of the package
Step1:  This notebook will generate a file in the current directory with the name "changelog_VERSION.md". You can edit and append this on front of the CHANGELOG file for the package release.
Step2:  Total commits by subpackage
Step3:  List Contributors
Step12:  Disaggregate by PR, Issue
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Python Code:
#  get date of last tag
from subprocess import Popen, PIPE
x, err = Popen('git log -1 --tags --simplify-by-decoration --pretty="%ai"| cat', stdin=PIPE, stdout=PIPE, stderr=PIPE, shell=True).communicate()
start_date = x.split()[0].decode('utf-8')
#  today's date
import datetime
release_date = str(datetime.datetime.today()).split()[0]
package_name = 'rvlib'
Explanation: rvlib Change Log Statistics
This notebook generates the summary statistics for a package. 
It assumes you are running this under the .ci directory at the toplevel of the package
End of explanation
from __future__ import print_function
import os
import json
import re
import sys
import pandas
from datetime import datetime, timedelta
from time import sleep
from subprocess import check_output
try:
    from urllib import urlopen
except:
    from urllib.request import urlopen
import ssl
import yaml
context = ssl._create_unverified_context()
CWD = os.path.abspath(os.path.curdir)
CWD
since_date = '--since="{start}"'.format(start=start_date)
since_date
since = datetime.strptime(start_date+" 0:0:0", "%Y-%m-%d %H:%M:%S")
since
# get __version__
f = "../{package}/version.py".format(package=package_name)
with open(f, 'r') as initfile:
     exec(initfile.readline())
   
Explanation: This notebook will generate a file in the current directory with the name "changelog_VERSION.md". You can edit and append this on front of the CHANGELOG file for the package release.
End of explanation
cmd = ['git', 'log', '--oneline', since_date]
ncommits = len(check_output(cmd).splitlines())
ncommits
Explanation: Total commits by subpackage
End of explanation
author_cmd = ['git', 'log', '--format=* %aN', since_date]
from collections import Counter
ncommits = len(check_output(cmd).splitlines())
all_authors = check_output(author_cmd).splitlines()
counter = Counter([author.decode('utf-8') for author in all_authors])
#        global_counter += counter
#        counters.update({'.'.join((package,subpackage)): counter})
unique_authors = sorted(set(all_authors))
unique_authors = counter.keys()
unique_authors
Explanation: List Contributors
End of explanation
from datetime import datetime, timedelta
ISO8601 = "%Y-%m-%dT%H:%M:%SZ"
PER_PAGE = 100
element_pat = re.compile(r'<(.+?)>')
rel_pat = re.compile(r'rel=[\'"](\w+)[\'"]')
def parse_link_header(headers):
    link_s = headers.get('link', '')
    urls = element_pat.findall(link_s)
    rels = rel_pat.findall(link_s)
    d = {}
    for rel,url in zip(rels, urls):
        d[rel] = url
    return d
def get_paged_request(url):
    get a full list, handling APIv3's paging
    results = []
    while url:
        #print("fetching %s" % url, file=sys.stderr)
        f = urlopen(url)
        results.extend(json.load(f))
        links = parse_link_header(f.headers)
        url = links.get('next')
    return results
def get_issues(project="QuantEcon/rvlib", state="closed", pulls=False):
    Get a list of the issues from the Github API.
    which = 'pulls' if pulls else 'issues'
    url = "https://api.github.com/repos/%s/%s?state=%s&per_page=%i" % (project, which, state, PER_PAGE)
    return get_paged_request(url)
def _parse_datetime(s):
    Parse dates in the format returned by the Github API.
    if s:
        return datetime.strptime(s, ISO8601)
    else:
        return datetime.fromtimestamp(0)
def issues2dict(issues):
    Convert a list of issues to a dict, keyed by issue number.
    idict = {}
    for i in issues:
        idict[i['number']] = i
    return idict
def is_pull_request(issue):
    Return True if the given issue is a pull request.
    return 'pull_request_url' in issue
def issues_closed_since(period=timedelta(days=365), project="QuantEcon/rvlib", pulls=False):
    Get all issues closed since a particular point in time. period
can either be a datetime object, or a timedelta object. In the
latter case, it is used as a time before the present.
    which = 'pulls' if pulls else 'issues'
    if isinstance(period, timedelta):
        period = datetime.now() - period
    url = "https://api.github.com/repos/%s/%s?state=closed&sort=updated&since=%s&per_page=%i" % (project, which, period.strftime(ISO8601), PER_PAGE)
    allclosed = get_paged_request(url)
    # allclosed = get_issues(project=project, state='closed', pulls=pulls, since=period)
    filtered = [i for i in allclosed if _parse_datetime(i['closed_at']) > period]
    # exclude rejected PRs
    if pulls:
        filtered = [ pr for pr in filtered if pr['merged_at'] ]
    return filtered
def sorted_by_field(issues, field='closed_at', reverse=False):
    Return a list of issues sorted by closing date date.
    return sorted(issues, key = lambda i:i[field], reverse=reverse)
def report(issues, show_urls=False):
    Summary report about a list of issues, printing number and title.
    
    # titles may have unicode in them, so we must encode everything below
    if show_urls:
        for i in issues:
            role = 'ghpull' if 'merged_at' in i else 'ghissue'
            print('* :%s:`%d`: %s' % (role, i['number'],
                                        i['title'].encode('utf-8')))
    else:
        for i in issues:
            print('* %d: %s' % (i['number'], i['title'].encode('utf-8')))
all_issues = {}
all_pulls = {}
total_commits = 0
#prj='pysal/libpysal'
prj = 'QuantEcon/rvlib'
issues = issues_closed_since(since, project=prj,pulls=False)
pulls = issues_closed_since(since, project=prj,pulls=True)
issues = sorted_by_field(issues, reverse=True)
pulls = sorted_by_field(pulls, reverse=True)
n_issues, n_pulls = map(len, (issues, pulls))
n_total = n_issues + n_pulls
issue_listing = []
for issue in issues:
    entry = "{title} (#{number})".format(title=issue['title'],number=issue['number'])
    issue_listing.append(entry)
pull_listing = []
for pull in pulls:
    entry = "{title} (#{number})".format(title=pull['title'],number=pull['number'])
    pull_listing.append(entry)
pull_listing
message = "We closed a total of {total} issues (enhancements and bug fixes) through {pr} pull requests".format(total=n_total, pr=n_pulls)
message = "{msg}, since our last release on {previous}.".format(msg=message, previous=str(start_date))
message
message += "\n\n## Issues Closed\n"
print(message)
issues = "\n".join(["  - "+issue for issue in issue_listing])
message += issues
message += "\n\n## Pull Requests\n"
pulls = "\n".join(["  - "+pull for pull in pull_listing])
message += pulls
print(message)
people = "\n".join(["  - "+person for person in unique_authors])
print(people)
message +="\n\nThe following individuals contributed to this release:\n\n{people}".format(people=people)
print(message)
import rvlib
v = rvlib.__version__
head = "# Changes\n\nVersion {version} ({release_date})\n\n".format(version=v, release_date=release_date)
print(head+message)
outfile = 'changelog.md'
with open(outfile, 'w') as of:
    of.write(head+message)
Explanation: Disaggregate by PR, Issue
End of explanation
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