Let's read code: the python-requests library

$ git clone https://github.com/
kennethreitz/requests
$ cd requests
$ subl requests (or editor of choice)
Optionally, feel free to follow along with your local
copy of the requests repo.
Slides:
http://www.slideshare.net/onceuponatimeforever/
lets-read-code
Let’s read code

Let’s read code:
python-requests library
Susan Tan
Twitter: @ArcTanSusan
PyTN 2016
February 7, 2016
Nashville, TN
2

First time you git clone and open
a new repo

This is a talk about
1. how to actively read a new Python codebase
2. reading thru the python-requests codebase
“Indeed, the ratio of time spent reading versus
writing is well over 10 to 1. We are constantly
reading old code as part of the effort to write
new code. ...[Therefore,] making it easy to read
makes it easier to write.”
Robert C. Martin, Clean Code: A Handbook of Agile Software Craftsmanship

Set up your editor to
• jump into any method or class definition
• search files by keywords
• get call hierarchy of any given method
or class
Note: I’ll be using Sublime Text with python-requests library.
Step 0: Prepare Your Editor

Step 1: Git clone and open the repo
$ git clone https://github.com/
kennethreitz/requests
$ cd requests
$ subl requests

Step 2: Set up local dev environment to
get into mindset of a contributor
python test_requests.py works too.
Source: http://docs.python-requests.org/en/latest/dev/todo/#development-dependencies

What it’s like to read a large codebase
Once you’ve set up your editor & local dev
environment…

Goal for today —
Figure out how this code snippet works
>>> r = requests.get('https://api.github.com/
user', auth=('user', 'pass'))
>>> r.status_code
200
>>> r.headers['content-type']
'application/json; charset=utf8'
>>> r.encoding
'utf-8'
>>> r.text
u'{"type":"User"...'
>>> r.json()
{u'private_gists': 419, u'total_private_repos':
77}

Step 3: Look at unit tests
• Over 1,700 lines of code in
test_requests.py. Where to look
first?
• Use git grep or keyword search
for “requests.get”

git grep requests.get test_requests.py
$ git grep requests.get test_requests.py
test_requests.py:95: requests.get
test_requests.py:103: requests.get('hiwpefhipowhefopw')
test_requests.py:105: requests.get('localhost:3128')
test_requests.py:107:
requests.get('localhost.localdomain:3128/')
test_requests.py:109: requests.get('10.122.1.1:3128/')
test_requests.py:111: requests.get('http://')
test_requests.py:187: r = requests.get(httpbin('redirect',
'1'))
test_requests.py:199: r = requests.get(httpbin('user-agent'),
headers=heads)
test_requests.py:207: r = requests.get(httpbin('get') + '?
test=true', params={'q': 'test'}, headers=heads)
test_requests.py:280: resp = requests.get(httpbin('redirect/
3'))
test_requests.py:289: resp = requests.get(httpbin('get'))
test_requests.py:291: resp = requests.get(httpbin('redirect/
1'))
......
$ git grep requests.get test_requests.py | wc -l
56

def test_DIGEST_HTTP_200_OK_GET(self, httpbin):
auth = HTTPDigestAuth('user', 'pass')
url = httpbin('digest-auth', ‘auth',
'user', 'pass')
r = requests.get(url, auth=auth)
assert r.status_code == 200
r = requests.get(url)
s = requests.session()
s.auth = HTTPDigestAuth('user', 'pass')
r = s.get(url)
test_requests.py
Looks like test
setup happens
here

url = httpbin('digest-auth', ‘auth', 'user',
'pass')
r = s.get(url)
test_requests.py
MUST be a
string type
401 means
forbidden. This is a
test to confirm auth
object is required.

'pass')
r = s.get(url)
test_requests.pyWhat’s a session?

'pass')
r = s.get(url)
test_requests.py
Let’s look at class definition

class HTTPDigestAuth(AuthBase):
"""Attaches HTTP Digest Authentication to the given Request
object."""
def __init__(self, username, password):
self.username = username
self.password = password
# Keep state in per-thread local storage
self._thread_local = threading.local()
def init_per_thread_state(self):
# Ensure state is initialized just once per-thread
…
def build_digest_header(self, method, url):
…
def handle_redirect(self, r, **kwargs):
…
def handle_401(self, r, **kwargs):
…
def __call__(self, r):
…
What is the HTTPDigestAuth class?
auth.py

Source: http://docs.python-requests.org/en/latest/user/authentication/#digest-
authentication
Again, requests has pretty good docs

'user', 'pass')
r = s.get(url)
test_requests.py
Let’s look at method definition

'user', 'pass')
test_requests.py
Let’s look at method definition
@pytest.fixture
def httpbin(httpbin):
httpbin_url = httpbin.url.rstrip('/') + '/'
def inner(*suffix):
return urljoin(httpbin_url, '/'.join(suffix))
return inner
This is httpbin() method:

Let's read code: the python-requests library

I’m still confused by what httpbin()
method is doing.
Next steps:
• look up “httpbin” in official request
docs.
• If that fails, then use a debugger.
We’ll do both.

Source:https://github.com/Runscope/httpbin

https://github.com/Runscope/
httpbin
Next Step: Let’s try out this http://
httpbin.org/.

http://httpbin.org/cookies
http://httpbin.org/get
Next Step: Let’s try out this http://httpbin.org/

httpbin’s /post/ endpoint is useful for testing requests
library
In [1]: import requests
In [2]: resp = requests.post('http://httpbin.org/post',
data={'name':'Susan'})
In [3]: resp.json()
Out[3]:
{u'args': {},
u'data': u'',
u'files': {},
u'form': {u'name': u'Susan'},
u'headers': {u'Accept': u'*/*',
u'Accept-Encoding': u'gzip, deflate',
u'Content-Length': u'10',
u'Content-Type': u'application/x-www-form-urlencoded',
u'Host': u'httpbin.org',
u'User-Agent': u'python-requests/2.9.1'},
u'json': None,
u'origin': u'50.148.141.36',
u'url': u'http://httpbin.org/post'}

• httpbin is everywhere in unit tests in the
requests repo every time a request is
made.
• This is a BIG step forward in our
understanding of unit tests in this repo.
httpbin endpoints and requests unit
test

pytest.set_trace()
Side Note: You can use pytest debugger
I placed a debugger in the httpbin method to see what the
output “url” looks like.

@pytest.fixture
def httpbin(httpbin):
# Issue #1483: Make sure the URL always has a
trailing slash
httpbin_url = httpbin.url.rstrip('/') + '/'
pytest.set_trace()
def inner(*suffix):
return urljoin(httpbin_url, '/'.join(suffix))
pytest.set_trace()
return inner
url = httpbin('digest-auth', 'auth', 'user',
'pass')
pytest.set_trace()
assert r.status_code == 200 test_requests.py

py.test test_requests.py::TestRequests::test_DIGEST_HTTP_200_OK_GET

What is this url http://127.0.0.01:73720/
digest-auth/auth/user/pass?!

What is httpbin.org/digest-auth/auth/user/pass?
Unit test gives me the answers to both fields. I type in
“user” and “pass” in both fields. The result?
✓

'user', 'pass')
r = s.get(url)
test_requests.py
Let’s look at
definition of
request method

def get(url, params=None, **kwargs):
"""Sends a GET request.
:param url: URL for the new :class:`Request` object.
:param params: (optional) Dictionary or bytes to be sent in the query
string for the :class:`Request`.
:param **kwargs: Optional arguments that ``request`` takes.
:return: :class:`Response <Response>` object
:rtype: requests.Response
"""
kwargs.setdefault('allow_redirects', True)
return request('get', url, params=params, **kwargs)
This is the get method
What’s happening here?
• Set default dict of key-value pairs to allow redirects by
default
• Returns a request. What’s a request? Look at method
above.
api.py

def request(method, url, **kwargs):
"""Constructs and sends a :class:`Request <Request>`.
:param method: method for the new :class:`Request` object.
:param url: URL for the new :class:`Request` object.
:param params: (optional) Dictionary or bytes to be sent in the query string for the :class:`Request`.
:param data: (optional) Dictionary, bytes, or file-like object to send in the body of the :class:`Request`.
:param json: (optional) json data to send in the body of the :class:`Request`.
:param headers: (optional) Dictionary of HTTP Headers to send with the :class:`Request`.
:param cookies: (optional) Dict or CookieJar object to send with the :class:`Request`.
:param files: (optional) Dictionary of ``'name': file-like-objects`` (or ``{'name': ('filename', fileobj)}``)
for multipart encoding upload.
:param auth: (optional) Auth tuple to enable Basic/Digest/Custom HTTP Auth.
:param timeout: (optional) How long to wait for the server to send data
before giving up, as a float, or a :ref:`(connect timeout, read
timeout) <timeouts>` tuple.
:type timeout: float or tuple
:param allow_redirects: (optional) Boolean. Set to True if POST/PUT/DELETE redirect following is allowed.
:type allow_redirects: bool
:param proxies: (optional) Dictionary mapping protocol to the URL of the proxy.
:param verify: (optional) whether the SSL cert will be verified. A CA_BUNDLE path can also be provided.
Defaults to ``True``.
:param stream: (optional) if ``False``, the response content will be immediately downloaded.
:param cert: (optional) if String, path to ssl client cert file (.pem). If Tuple, ('cert', 'key') pair.
:return: :class:`Response <Response>` object
:rtype: requests.Response
Usage::
>>> import requests
>>> req = requests.request('GET', 'http://httpbin.org/get')
<Response [200]>
"""
# By using the 'with' statement we are sure the session is closed, thus we
# avoid leaving sockets open which can trigger a ResourceWarning in some
# cases, and look like a memory leak in others.
with sessions.Session() as session:
return session.request(method=method, url=url, **kwargs)
api.py
This is the request method

What are sessions??
api.py
This is the same request method without
docstrings or comments

class Session(SessionRedirectMixin):
"""A Requests session.
Provides cookie persistence, connection-pooling, and configuration.
Basic Usage::
>>> import requests
>>> s = requests.Session()
>>> s.get('http://httpbin.org/get')
<Response [200]>
Or as a context manager::
>>> with requests.Session() as s:
>>> s.get('http://httpbin.org/get')
<Response [200]>
"""
__attrs__ = [
'headers', 'cookies', 'auth', 'proxies', 'hooks', 'params', 'verify',
'cert', 'prefetch', 'adapters', 'stream', 'trust_env',
'max_redirects',
]
def __init__(self):
#: A case-insensitive dictionary of headers to be sent on each
#: :class:`Request <Request>` sent from this
#: :class:`Session <Session>`.
self.headers = default_headers()
#: Default Authentication tuple or object to attach to
#: :class:`Request <Request>`.
self.auth = None
#: Dictionary mapping protocol or protocol and host to the URL of the proxy
#: (e.g. {'http': 'foo.bar:3128', 'http://host.name': 'foo.bar:4012'}) to
#: be used on each :class:`Request <Request>`.
self.proxies = {}
#: Event-handling hooks.
self.hooks = default_hooks()
#: Dictionary of querystring data to attach to each
#: :class:`Request <Request>`. The dictionary values may be lists for
#: representing multivalued query parameters.
self.params = {}
#: Stream response content default.
self.stream = False
#: SSL Verification default.
self.verify = True
#: SSL certificate default.
self.cert = None
….
sessions.py
R
eally
long
class
definition
of“Sessions”

What’s a session?
• an object that persists parameters across
requests
• makes use of urllib3’s connection pooling
• has all methods of request API
• provides default data to request object
• note: requests has well written docs

What is this request() in sessions?
api.py
This is the same request method without
docstrings or comments

sessions.py
def request(self, method, url,
params=None,
data=None,
headers=None,
cookies=None,
files=None,
auth=None,
timeout=None,
allow_redirects=True,
proxies=None,
hooks=None,
stream=None,
verify=None,
cert=None,
json=None):
# Create the Request.
req = Request(
method = method.upper(),
url = url,
headers = headers,
files = files,
data = data or {},
json = json,
params = params or {},
auth = auth,
cookies = cookies,
hooks = hooks,
)
prep = self.prepare_request(req)
proxies = proxies or {}
settings = self.merge_environment_settings(
prep.url, proxies, stream, verify, cert
)
# Send the request.
send_kwargs = {
'timeout': timeout,
'allow_redirects': allow_redirects,
}
send_kwargs.update(settings)
resp = self.send(prep, **send_kwargs)
return resp
What is this request() in sessions?

req = Request(
url = url,
headers = headers,
files = files,
data = data or {},
json = json,
auth = auth,
cookies = cookies,
hooks = hooks,
)
params=None,
data=None,
headers=None,
cookies=None,
files=None,
auth=None,
timeout=None,
proxies=None,
hooks=None,
stream=None,
verify=None,
cert=None,
json=None):
req = Request(
url = url,
headers = headers,
files = files,
data = data or {},
json = json,
auth = auth,
cookies = cookies,
hooks = hooks,
)
)
# Send the request.
send_kwargs = {
'timeout': timeout,
}
return resp
sessions.py
What’s happening in this
code?
1. create request
2. create prepare request
object “prep”
3. send request
4. return response
1
2
3
4

1.create request
Let’s dissect requests/sessions.py

class Request(RequestHooksMixin):
"""A user-created :class:`Request <Request>` object.
Used to prepare a :class:`PreparedRequest <PreparedRequest>`, which is sent to the server.
:param method: HTTP method to use.
:param url: URL to send.
:param headers: dictionary of headers to send.
:param files: dictionary of {filename: fileobject} files to multipart upload.
:param data: the body to attach to the request. If a dictionary is provided, form-encoding will take place.
:param json: json for the body to attach to the request (if files or data is not specified).
:param params: dictionary of URL parameters to append to the URL.
:param auth: Auth handler or (user, pass) tuple.
:param cookies: dictionary or CookieJar of cookies to attach to this request.
:param hooks: dictionary of callback hooks, for internal usage.
Usage::
>>> import requests
>>> req = requests.Request('GET', 'http://httpbin.org/get')
>>> req.prepare()
<PreparedRequest [GET]>
"""
def __init__(self, method=None, url=None, headers=None, files=None,
data=None, params=None, auth=None, cookies=None, hooks=None, json=None):
# Default empty dicts for dict params.
data = [] if data is None else data
files = [] if files is None else files
headers = {} if headers is None else headers
params = {} if params is None else params
hooks = {} if hooks is None else hooks
for (k, v) in list(hooks.items()):
self.register_hook(event=k, hook=v)
self.method = method
self.url = url
self.headers = headers
self.files = files
self.data = data
self.json = json
self.params = params
self.auth = auth
self.cookies = cookies
def __repr__(self):
return '<Request [%s]>' % (self.method)
def prepare(self):
"""Constructs a :class:`PreparedRequest <PreparedRequest>` for transmission and returns it."""
p = PreparedRequest()
p.prepare(
method=self.method,
url=self.url,
headers=self.headers,
files=self.files,
data=self.data,
json=self.json,
params=self.params,
auth=self.auth,
cookies=self.cookies,
hooks=self.hooks,
)
return p
models.py
request arguments
that were passed in
This is Requests
class definition

params=None,
data=None,
headers=None,
cookies=None,
files=None,
auth=None,
timeout=None,
proxies=None,
hooks=None,
stream=None,
verify=None,
cert=None,
json=None):
req = Request(
url = url,
headers = headers,
files = files,
data = data or {},
json = json,
auth = auth,
cookies = cookies,
hooks = hooks,
)
)
# Send the request.
send_kwargs = {
'timeout': timeout,
}
return resp
sessions.py
code?
1. create request
object “prep”
3. send request
4. return response1
2
3
4
✓
prep =
self.prepare_request(
req)

object “prep”

def prepare_request(self, request):
"""Constructs a :class:`PreparedRequest <PreparedRequest>` for
transmission and returns it. The :class:`PreparedRequest` has settings
merged from the :class:`Request <Request>` instance and those of the
:class:`Session`.
:param request: :class:`Request` instance to prepare with this
session's settings.
"""
cookies = request.cookies or {}
# Bootstrap CookieJar.
if not isinstance(cookies, cookielib.CookieJar):
cookies = cookiejar_from_dict(cookies)
# Merge with session cookies
merged_cookies = merge_cookies(
merge_cookies(RequestsCookieJar(), self.cookies), cookies)
# Set environment's basic authentication if not explicitly set.
auth = request.auth
if self.trust_env and not auth and not self.auth:
auth = get_netrc_auth(request.url)
p = PreparedRequest()
p.prepare(
method=request.method.upper(),
url=request.url,
files=request.files,
data=request.data,
json=request.json,
headers=merge_setting(request.headers, self.headers,
dict_class=CaseInsensitiveDict),
params=merge_setting(request.params, self.params),
auth=merge_setting(auth, self.auth),
cookies=merged_cookies,
hooks=merge_hooks(request.hooks, self.hooks),
)
return p
What is
“prepare_
request()”?
sessions.py
setup stuff happens
“PreparedRequests class”? What
is the class method “prepare()”?

class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
"""The fully mutable :class:`PreparedRequest <PreparedRequest>` object,
containing the exact bytes that will be sent to the server.
Generated from either a :class:`Request <Request>` object or manually.
Usage::
>>> import requests
>>> req = requests.Request('GET', 'http://httpbin.org/get')
>>> r = req.prepare()
<PreparedRequest [GET]>
>>> s.send(r)
<Response [200]>
"""
def __init__(self):
#: HTTP verb to send to the server.
self.method = None
#: HTTP URL to send the request to.
self.url = None
#: dictionary of HTTP headers.
self.headers = None
# The `CookieJar` used to create the Cookie header will be stored here
# after prepare_cookies is called
self._cookies = None
#: request body to send to the server.
self.body = None
#: dictionary of callback hooks, for internal usage.
def prepare(self, method=None, url=None, headers=None, files=None,
data=None, params=None, auth=None, cookies=None, hooks=None, json=None):
"""Prepares the entire request with the given parameters."""
self.prepare_method(method)
self.prepare_url(url, params)
self.prepare_headers(headers)
self.prepare_cookies(cookies)
self.prepare_body(data, files, json)
self.prepare_auth(auth, url)
# Note that prepare_auth must be last to enable authentication schemes
# such as OAuth to work on a fully prepared request.
# This MUST go after prepare_auth. Authenticators could add a hook
self.prepare_hooks(hooks)
models.py
W
hat are
Prepared
Requests?
Lots more
layers of
abstraction!

params=None,
data=None,
headers=None,
cookies=None,
files=None,
auth=None,
timeout=None,
proxies=None,
hooks=None,
stream=None,
verify=None,
cert=None,
json=None):
req = Request(
url = url,
headers = headers,
files = files,
data = data or {},
json = json,
auth = auth,
cookies = cookies,
hooks = hooks,
)
)
# Send the request.
send_kwargs = {
'timeout': timeout,
}
return resp
sessions.py
code?
1. create request
object “prep”
3. send request
4. return response1
2
3
4
✓
✓
resp =
self.send(prep,
**send_kwargs)
return resp

3. send request
4. return response
def send(self, request, **kwargs):
"""Send a given PreparedRequest."""
….[LONG METHOD DEFINITION HERE]….
return r
send()request object response object
sessions.py

def send(self, request, **kwargs):
"""Send a given PreparedRequest."""
# Set defaults that the hooks can utilize to ensure they
always have
# the correct parameters to reproduce the previous request.
kwargs.setdefault('stream', self.stream)
kwargs.setdefault('verify', self.verify)
kwargs.setdefault('cert', self.cert)
kwargs.setdefault('proxies', self.proxies)
# It's possible that users might accidentally send a Request
object.
# Guard against that specific failure case.
if not isinstance(request, PreparedRequest):
raise ValueError('You can only send PreparedRequests.')
checked_urls = set()
while request.url in self.redirect_cache:
checked_urls.add(request.url)
new_url = self.redirect_cache.get(request.url)
if new_url in checked_urls:
break
request.url = new_url
# Set up variables needed for resolve_redirects and
dispatching of hooks
allow_redirects = kwargs.pop('allow_redirects', True)
stream = kwargs.get('stream')
hooks = request.hooks
# Get the appropriate adapter to use
adapter = self.get_adapter(url=request.url)
# Start time (approximately) of the request
start = datetime.utcnow()
# Send the request
r = adapter.send(request, **kwargs)
# Total elapsed time of the request (approximately)
r.elapsed = datetime.utcnow() - start
# Response manipulation hooks
r = dispatch_hook('response', hooks, r, **kwargs)
# Persist cookies
if r.history:
# If the hooks create history then we want those cookies
too
for resp in r.history:
extract_cookies_to_jar(self.cookies, resp.request,
resp.raw)
extract_cookies_to_jar(self.cookies, request, r.raw)
# Redirect resolving generator.
gen = self.resolve_redirects(r, request, **kwargs)
# Resolve redirects if allowed.
history = [resp for resp in gen] if allow_redirects else []
# Shuffle things around if there's history.
if history:
# Insert the first (original) request at the start
history.insert(0, r)
# Get the last request made
r = history.pop()
r.history = history
if not stream:
r.content
return r
3. send request
4. return response
# Get the appropriate adapter to use
adapter = self.get_adapter(url=request.url)
# Start time (approximately) of the request
start = datetime.utcnow()
# Send the request
r = adapter.send(request, **kwargs)
sessions.py
What is the send method
doing in adapters.py?
What’s an adapter?

What’s an adapter?
“Transport Adapters provide a mechanism to
define interaction methods for an HTTP service.
Requests ships with a single Transport
Adapter, the HTTPAdapter. This adapter
provides the default Requests interaction with
HTTP and HTTPS using the powerful urllib3
library. Whenever a Requests session is
initialized, one of these is attached to the
session object for HTTP, and one for HTTPS.”
—“Transport Adapters” in requests advanced docs

class HTTPAdapter(BaseAdapter):
"""
The built-in HTTP Adapter for urllib3.
Provides a general-case interface for Requests sessions to contact HTTP and
HTTPS urls by implementing the Transport Adapter interface. This class will
usually be created by the :class:`Session <Session>` class under the
covers.
:param pool_connections: The number of urllib3 connection pools to cache.
:param pool_maxsize: The maximum number of connections to save in the pool.
:param int max_retries: The maximum number of retries each connection
should attempt. Note, this applies only to failed DNS lookups, socket
connections and connection timeouts, never to requests where data has
made it to the server. By default, Requests does not retry failed
connections. If you need granular control over the conditions under
which we retry a request, import urllib3's ``Retry`` class and pass
that instead.
:param pool_block: Whether the connection pool should block for connections.
Usage::
>>> import requests
>>> a = requests.adapters.HTTPAdapter(max_retries=3)
>>> s.mount('http://', a)
"""
This is HTTPAdapter class, the interface for urllib3
adapters.py

def send(self, request, stream=False, timeout=None, verify=True, cert=None, proxies=None):
"""Sends PreparedRequest object. Returns Response object.
:param request: The :class:`PreparedRequest <PreparedRequest>` being sent.
:param stream: (optional) Whether to stream the request content.
:param timeout: (optional) How long to wait for the server to send
data before giving up, as a float, or a :ref:`(connect timeout,
read timeout) <timeouts>` tuple.
:type timeout: float or tuple
:param verify: (optional) Whether to verify SSL certificates.
:param cert: (optional) Any user-provided SSL certificate to be trusted.
:param proxies: (optional) The proxies dictionary to apply to the request.
"""
conn = self.get_connection(request.url, proxies)
self.cert_verify(conn, request.url, verify, cert)
url = self.request_url(request, proxies)
self.add_headers(request)
chunked = not (request.body is None or 'Content-Length' in request.headers)
if isinstance(timeout, tuple):
try:
connect, read = timeout
timeout = TimeoutSauce(connect=connect, read=read)
except ValueError as e:
# this may raise a string formatting error.
err = ("Invalid timeout {0}. Pass a (connect, read) "
"timeout tuple, or a single float to set "
"both timeouts to the same value".format(timeout))
raise ValueError(err)
else:
timeout = TimeoutSauce(connect=timeout, read=timeout)
try:
if not chunked:
resp = conn.urlopen(
method=request.method,
url=url,
body=request.body,
headers=request.headers,
redirect=False,
assert_same_host=False,
preload_content=False,
decode_content=False,
retries=self.max_retries,
timeout=timeout
)
# Send the request.
else:
if hasattr(conn, 'proxy_pool'):
conn = conn.proxy_pool
low_conn = conn._get_conn(timeout=DEFAULT_POOL_TIMEOUT)
try:
low_conn.putrequest(request.method,
url,
skip_accept_encoding=True)
for header, value in request.headers.items():
low_conn.putheader(header, value)
low_conn.endheaders()
for i in request.body:
low_conn.send(hex(len(i))[2:].encode('utf-8'))
low_conn.send(b'rn')
low_conn.send(i)
low_conn.send(b'rn')
low_conn.send(b'0rnrn')
# Receive the response from the server
try:
# For Python 2.7+ versions, use buffering of HTTP
# responses
r = low_conn.getresponse(buffering=True)
except TypeError:
# For compatibility with Python 2.6 versions and back
r = low_conn.getresponse()
resp = HTTPResponse.from_httplib(
r,
pool=conn,
connection=low_conn,
preload_content=False,
decode_content=False
)
except:
# If we hit any problems here, clean up the connection.
# Then, reraise so that we can handle the actual exception.
low_conn.close()
raise
except (ProtocolError, socket.error) as err:
raise ConnectionError(err, request=request)
except MaxRetryError as e:
if isinstance(e.reason, ConnectTimeoutError):
# TODO: Remove this in 3.0.0: see #2811
if not isinstance(e.reason, NewConnectionError):
raise ConnectTimeout(e, request=request)
if isinstance(e.reason, ResponseError):
raise RetryError(e, request=request)
raise ConnectionError(e, request=request)
except ClosedPoolError as e:
raise ConnectionError(e, request=request)
except _ProxyError as e:
raise ProxyError(e)
except (_SSLError, _HTTPError) as e:
if isinstance(e, _SSLError):
raise SSLError(e, request=request)
elif isinstance(e, ReadTimeoutError):
raise ReadTimeout(e, request=request)
else:
raise
adapters.py
Size
5
font. This
is
the
definition
of send() m
ethod. Over
100
lines
long
and
it can’t fit this
slide.
Exercise
left to
reader to
read
thru
this
send() m
ethod.

"""
requests.adapters
~~~~~~~~~~~~~~~~~
This module contains the transport adapters that Requests uses to define
and maintain connections.
"""
import os.path
import socket
from .models import Response
from .packages.urllib3.poolmanager import PoolManager, proxy_from_url
from .packages.urllib3.response import HTTPResponse
from .packages.urllib3.util import Timeout as TimeoutSauce
from .packages.urllib3.util.retry import Retry
from .compat import urlparse, basestring
from .utils import (DEFAULT_CA_BUNDLE_PATH, get_encoding_from_headers,
prepend_scheme_if_needed, get_auth_from_url, urldefragauth,
select_proxy)
from .structures import CaseInsensitiveDict
from .packages.urllib3.exceptions import ClosedPoolError
from .packages.urllib3.exceptions import ConnectTimeoutError
from .packages.urllib3.exceptions import HTTPError as _HTTPError
from .packages.urllib3.exceptions import MaxRetryError
from .packages.urllib3.exceptions import NewConnectionError
from .packages.urllib3.exceptions import ProxyError as _ProxyError
from .packages.urllib3.exceptions import ProtocolError
from .packages.urllib3.exceptions import ReadTimeoutError
from .packages.urllib3.exceptions import SSLError as _SSLError
from .packages.urllib3.exceptions import ResponseError
from .cookies import extract_cookies_to_jar
from .exceptions import (ConnectionError, ConnectTimeout, ReadTimeout, SSLError,
ProxyError, RetryError)
from .auth import _basic_auth_str
DEFAULT_POOLBLOCK = False
DEFAULT_POOLSIZE = 10
DEFAULT_RETRIES = 0
DEFAULT_POOL_TIMEOUT = None
the imports at top of requests/adapters.py
urllib3 is imported here
adapters.py

def send(self, request, stream=False, timeout=None, verify=True, cert=None,
proxies=None):
…
import pytest
pytest.set_trace()
return self.build_response(request, resp)
Let’s place a debugger in
adapters.py and run the same unit
test again.
py.test test_requests.py::TestRequests::test_DIGEST_HTTP_200_OK_GET
Run this same unit test on command line
adapters.py

Use pytest debugger to see output of send() method
[48] > /Users/susantan/Projects/requests/requests/
adapters.py(455)send()
-> return self.build_response(request, resp)
(Pdb++) request.url
‘http://127.0.0.1:58948/digest-auth/auth/user/pass'
(Pdb++) resp
<requests.packages.urllib3.response.HTTPResponse object
at 0x102c15110>
(Pdb++) our_version_of_response_object =
self.build_response(request, resp)
(Pdb++) our_version_of_response_object.json()
{u'authenticated': True, u'user': u’user'}
(Pdb++) our_version_of_response_object.status_code
200

In summary, how does “request.get()”
work?

'user', 'pass')
r = s.get(url)
test_requests.py

return session.request(method=method, url=url,
**kwargs)
api.py
This is the request method
This is the get method
def get(url, params=None, **kwargs):
kwargs.setdefault('allow_redirects', True)
return request('get', url, params=params,
**kwargs)

params=None,
data=None,
headers=None,
cookies=None,
files=None,
auth=None,
timeout=None,
proxies=None,
hooks=None,
stream=None,
verify=None,
cert=None,
json=None):
req = Request(
url = url,
headers = headers,
files = files,
data = data or {},
json = json,
auth = auth,
cookies = cookies,
hooks = hooks,
)
)
# Send the request.
send_kwargs = {
'timeout': timeout,
}
return resp
sessions.py
code?
1. create request
object “prep”
3. send request
4. return response1
2
3
4
✓
✓
✓
✓

A mental map of files and associated
function calls
adapters.py
sessions.py
models.py
api.py
File names
test_requests.py
request(), get(), session.request()
class Request(), class
PreparedRequest()
class Request(), prepare_request(), send()
send()
method or class names
test_DIGEST_HTTP_200_OK_GET()

No walkthrough of a codebase is the same
for any person. ie: skip reading any unit test.
In [4]: import requests
In [4]: resp = requests.post('http://httpbin.org/post',
data={'name':'Susan'})
[14] > /Users/susantan/Projects/requests/requests/
adapters.py(346)send()
-> conn = self.get_connection(request.url, proxies)
(Pdb++) l
341 """
342
343 import pytest
344 pytest.set_trace()
345
346 -> conn = self.get_connection(request.url, proxies)
347
348 self.cert_verify(conn, request.url, verify, cert)
349 url = self.request_url(request, proxies)
350 self.add_headers(request)
351
(Pdb++) dir()
['cert', 'proxies', 'pytest', 'request', 'self', 'stream', 'timeout',
'verify']
Set breakpoints!

• What did we do? We took a code snippet
(“requests.get(url)”)& its unit test and followed it
through.
• We really know “requests.get(url)” works in great
depth.
• We chose a small code snippet to guide our
walkthrough.
• We set breakpoints, read unit tests, & read some
docs.
• Ask LOTS of questions & find the answers. Be
curious.
Takeaways

• Ideally, get contact info of core devs & get in
touch with them.
• Ask on mailing list, chat, IRC, Twitter, in-person.
• Use git blame to see who wrote that section.
• File an issue for better docs, better docstrings &
code comments.
What to do when you get
really stuck on figuring out
codebase?

Related Past Talk on Reading Code
• RailsConf 2014 - Reading Code Good by Saron
Yitbarek @saronyitbarek: about experiences with
a weekly code club that would read a code base
as a group

Hope this helps. Thanks for attending!
Where to reach me:
@ArcTanSusan on Twitter, IRC
San Francisco, CA

Let's read code: the python-requests library

More Related Content

What's hot (20)

Similar to Let's read code: the python-requests library (20)

More from Susan Tan (6)

Recently uploaded (20)

Let's read code: the python-requests library