Latest posts for tag python
A little gitpython recipe to list the paths of all files in a commit:
#!/usr/bin/python3 import git from pathlib import Path import sys def list_paths(root_tree, path=Path(".")): for blob in root_tree.blobs: yield path / blob.name for tree in root_tree.trees: yield from list_paths(tree, path / tree.name) repo = git.Repo(".", search_parent_directories=True) commit = repo.commit(sys.argv[1]) for path in list_paths(commit.tree): print(path)
It can be a good base, for example, for writing a script that, given two git branches, shows which django migrations are in one and not in the other, without doing any git checkout of the code.
One of the software I maintain for work is a GUI data browser that uses Tornado as a backend and a web browser as a front-end.
It is quite convenient to start the command and have the browser open automatically on the right URL. It's quite annoying to start the command and be told that the default port is already in use.
I've needed this trick quite often, also when writing unit tests, and it's time I note it down somewhere, so it's easier to find than going through Tornado's unittest code where I found it the first time.
This is how to start Tornado on a free random port:
from tornado.options import define, options import tornado.netutil import tornado.httpserver define("web_port", type=int, default=None, help="listening port for web interface") application = Application(self.db_url) if options.web_port is None: sockets = tornado.netutil.bind_sockets(0, '127.0.0.1') self.web_port = sockets[0].getsockname()[:2][1] server = tornado.httpserver.HTTPServer(application) server.add_sockets(sockets) else: server = tornado.httpserver.HTTPServer(application) server.listen(options.web_port)
I am writing a little application server for microservices written as compiled
binaries, and I would like to log execution statistics from getrusage(2).
The application server is written using asyncio, and processes are managed using asyncio subprocesses.
Unfortunately, asyncio uses os.waitpid instead of os.wait4 to reap child
processes, and to get rusage information one has to delve into the asyncio
innards, and provide a custom ChildWatcher implementation. Here's how I did
it:
import asyncio from asyncio.log import logger from contextlib import contextmanager import os class ExtendedResults: def __init__(self): self.rusage = None self.returncode = None class SafeChildWatcherWithRusage(asyncio.SafeChildWatcher): """ SafeChildWatcher that uses os.wait4 to also get rusage information. """ rusage_results = {} @classmethod @contextmanager def monitor(cls, proc): """ Return an ExtendedResults that gets filled when the process exits """ assert proc.pid > 0 pid = proc.pid extended_results = ExtendedResults() cls.rusage_results[pid] = extended_results try: yield extended_results finally: cls.rusage_results.pop(pid, None) def _do_waitpid(self, expected_pid): # The original is in asyncio/unix_events.py; on new python versions, it # makes sense to check changes to it and port them here assert expected_pid > 0 try: pid, status, rusage = os.wait4(expected_pid, os.WNOHANG) except ChildProcessError: # The child process is already reaped # (may happen if waitpid() is called elsewhere). pid = expected_pid returncode = 255 logger.warning( "Unknown child process pid %d, will report returncode 255", pid) else: if pid == 0: # The child process is still alive. return returncode = self._compute_returncode(status) if self._loop.get_debug(): logger.debug('process %s exited with returncode %s', expected_pid, returncode) extended_results = self.rusage_results.get(pid) if extended_results is not None: extended_results.rusage = rusage extended_results.returncode = returncode try: callback, args = self._callbacks.pop(pid) except KeyError: # pragma: no cover # May happen if .remove_child_handler() is called # after os.waitpid() returns. if self._loop.get_debug(): logger.warning("Child watcher got an unexpected pid: %r", pid, exc_info=True) else: callback(pid, returncode, *args) @classmethod def install(cls): loop = asyncio.get_event_loop() child_watcher = cls() child_watcher.attach_loop(loop) asyncio.set_child_watcher(child_watcher)
To use it:
from .hacks import SafeChildWatcherWithRusage SafeChildWatcherWithRusage.install() ... @coroutine def run(self, *args, **kw): kw["stdin"] = asyncio.subprocess.PIPE kw["stdout"] = asyncio.subprocess.PIPE kw["stderr"] = asyncio.subprocess.PIPE self.started = time.time() self.proc = yield from asyncio.create_subprocess_exec(*args, **kw) from .hacks import SafeChildWatcherWithRusage with SafeChildWatcherWithRusage.monitor(self.proc) as results: yield from asyncio.tasks.gather( self.write_stdin(self.proc.stdin), self.read_stdout(self.proc.stdout), self.read_stderr(self.proc.stderr) ) self.returncode = yield from self.proc.wait() self.rusage = results.rusage self.ended = time.time()
Python mailbox.mbox is not good at opening compressed mailboxes:
>>> import mailbox >>> print(len(mailbox.mbox("/tmp/test.mbox"))) 9 >>> print(len(mailbox.mbox("/tmp/test.mbox.gz"))) 0 >>> print(len(mailbox.mbox("/tmp/test1.mbox.xz"))) 0
For a prototype rewrite of the MIA team's Echelon (the engine behind mia-query), I needed to scan compressed mailboxes, and I had to work around this limitation.
Here is the alternative mailbox.mbox implementation:
import lzma import gzip import bz2 import mailbox class StreamMbox(mailbox.mbox): """ mailbox.mbox does not support opening a stream, which is sad. This is a subclass that works around it """ def __init__(self, fd: BinaryIO, factory=None, create: bool = True): # Do not call parent __init__, just redo everything here to be able to # open a stream. This will need to be re-reviewed for every new version # of python's stdlib. # Mailbox constructor self._path = None self._factory = factory # _singlefileMailbox constructor self._file = fd self._toc = None self._next_key = 0 self._pending = False # No changes require rewriting the file. self._pending_sync = False # No need to sync the file self._locked = False self._file_length = None # Used to record mailbox size # mbox constructor self._message_factory = mailbox.mboxMessage def flush(self): raise NotImplementedError("StreamMbox is a readonly class") class UsageExample: DECOMPRESS = { ".xz": lzma.open, ".gz": gzip.open, ".bz2": bz2.open, } @classmethod def scan(cls, path: Path) -> Generator[ScannedEmail, None, None]: decompress = cls.DECOMPRESS.get(path.suffix) if decompress is None: with open(path.as_posix(), "rb") as fd: yield from cls.scan_fd(path, fd) else: with decompress(path.as_posix(), "rb") as fd: yield from cls.scan_fd(path, fd) @classmethod def scan_fd(cls, path: Path, fd: BinaryIO) -> Generator[ScannedEmail, None, None]: mbox = StreamMbox(fd) for msg in mbox: ...
This code:
#!/usr/bin/python3 class Test: def __init__(self, items=[]): self.items = items def add(self, item): self.items.append(item) a = Test() a.add("foo") b = Test() b.add("bar") print(repr(a.items)) print(repr(b.items))
"obviously" prints:
['foo', 'bar'] ['foo', 'bar']
Because the default value of the items argument is a mutable list constructed
just once when the code is
compiled when the function definition is evaluated, and then reused.
So, in Python, mutable items in default arguments are a good way to get more fun time with debugging.
I am exposing some library functions using a TurboGears2 controller (see Building a web-based API with Turbogears2). It turns out that some functions return a dict, some a list, some a string, and TurboGears 2 only allows JSON serialisation for dicts.
A simple work-around for this is to wrap the function result into a dict, something like this:
@expose("json") @validate(validator_dispatcher, error_handler=api_validation_error) def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return dict(r=res)
It would be nice, however, to have an @webapi() decorator that
automatically wraps the function result with the dict:
def webapi(func): def dict_wrap(*args, **kw): return dict(r=func(*args, **kw)) return dict_wrap # ...in the controller... @expose("json") @validate(validator_dispatcher, error_handler=api_validation_error) @webapi def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return res
This works, as long as @webapi appears last in the list of decorators.
This is because if it appears last it will be the first to wrap the function,
and so it will not interfere with the tg.decorators machinery.
Would it be possible to create a decorator that can be put anywhere among the decorator list? Yes, it is possible but tricky, and it gives me the feeling that it may break in any future version of TurboGears:
class webapi(object): def __call__(self, func): def dict_wrap(*args, **kw): return dict(r=func(*args, **kw)) # Migrate the decoration attribute to our new function if hasattr(func, 'decoration'): dict_wrap.decoration = func.decoration dict_wrap.decoration.controller = dict_wrap delattr(func, 'decoration') return dict_wrap # ...in the controller... @expose("json") @validate(validator_dispatcher, error_handler=api_validation_error) @webapi def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return res
As a convenience, TurboGears 2 offers, in the decorators module, a way to
build decorator "hooks":
class before_validate(_hook_decorator): '''A list of callables to be run before validation is performed''' hook_name = 'before_validate' class before_call(_hook_decorator): '''A list of callables to be run before the controller method is called''' hook_name = 'before_call' class before_render(_hook_decorator): '''A list of callables to be run before the template is rendered''' hook_name = 'before_render' class after_render(_hook_decorator): '''A list of callables to be run after the template is rendered. Will be run before it is returned returned up the WSGI stack''' hook_name = 'after_render'
The way these are invoked can be found in the _perform_call function in
tg/controllers.py.
To show an example use of those hooks, let's add a some polygen wisdom to every data structure we return:
class wisdom(decorators.before_render): def __init__(self, grammar): super(wisdom, self).__init__(self.add_wisdom) self.grammar = grammar def add_wisdom(self, remainder, params, output): from subprocess import Popen, PIPE output["wisdom"] = Popen(["polyrun", self.grammar], stdout=PIPE).communicate()[0] # ...in the controller... @wisdom("genius") @expose("json") @validate(validator_dispatcher, error_handler=api_validation_error) def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return res
These hooks cannot however be used for what I need, that is, to wrap the result inside a dict. The reason is because they are called in this way:
controller.decoration.run_hooks( 'before_render', remainder, params, output)
and not in this way:
output = controller.decoration.run_hooks( 'before_render', remainder, params, output)
So it is possible to modify the output (if it is a mutable structure) but not to exchange it with something else.
Can we do even better? Sure we can. We can assimilate @expose and @validate
inside @webapi to avoid repeating those same many decorator lines over and
over again:
class webapi(object): def __init__(self, error_handler = None): self.error_handler = error_handler def __call__(self, func): def dict_wrap(*args, **kw): return dict(r=func(*args, **kw)) res = expose("json")(dict_wrap) res = validate(validator_dispatcher, error_handler=self.error_handler)(res) return res # ...in the controller... @expose("json") def api_validation_error(self, **kw): pylons.response.status = "400 Error" return dict(e="validation error on input fields", form_errors=pylons.c.form_errors) @webapi(error_handler=api_validation_error) def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return res
This got rid of @expose and @validate, and provides almost all the
default values that I need. Unfortunately I could not find out how to access
api_validation_error from the decorator so that I can pass it to the
validator, therefore I remain with the inconvenience of having to explicitly
pass it every time.
I am using TurboGears2 to export a python API over the web. Every API method is wrapper by a controller method that validates the parameters and returns the results encoded in JSON.
The basic idea is this:
@expose("json") def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return res
To validate the parameters we can use forms, it's their job after all:
class ListColoursForm(TableForm): fields = [ # One field per parameter twf.TextField("filter", help_text="Please enter the string to use as a filter"), twf.TextField("productID", help_text="Please enter the product ID"), twf.TextField("maxResults", validator=twfv.Int(min=0), default=200, size=5, help_text="Please enter the maximum number of results"), ] list_colours_form=ListColoursForm() #... @expose("json") @validate(list_colours_form, error_handler=list_colours_validation_error) def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Parameter validation is done by the form # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return res
All straightforward so far. However, this means that we need two exposed methods for every API call: one for the API call and one error handler. For every API call, we have to type the name several times, which is error prone and risks to get things mixed up.
We can however have a single error handler for all methonds:
def get_method(): ''' The method name is the first url component after the controller name that does not start with 'test' ''' found_controller = False for name in pylons.c.url.split("/"): if not found_controller and name == "controllername": found_controller = True continue if name.startswith("test"): continue if found_controller: return name return None class ValidatorDispatcher: ''' Validate using the right form according to the value of the "method" field ''' def validate(self, args, state): method = args.get("method", None) # Extract the method from the URL if it is missing if method is None: method = get_method() args["method"] = method return forms[method].validate(args, state) validator_dispatcher = ValidatorDispatcher()
This validator will try to find the method name, either as a form field
or by parsing the URL. It will then use the method name to find the form to use
for validation, and pass control to the validate method of that form.
We then need to add an extra "method" field to our forms, and arrange the forms inside a dictionary:
class ListColoursForm(TableForm): fields = [ # One hidden field to have a place for the method name twf.HiddenField("method") # One field per parameter twf.TextField("filter", help_text="Please enter the string to use as a filter"), #... forms["list_colours"] = ListColoursForm()
And now our methods become much nicer to write:
@expose("json") def api_validation_error(self, **kw): pylons.response.status = "400 Error" return dict(form_errors=pylons.c.form_errors) @expose("json") @validate(validator_dispatcher, error_handler=api_validation_error) def list_colours(self, filter=None, productID=None, maxResults=100, **kw): # Parameter validation is done by the form # Call API res = self.engine.list_colours(filter, productID, maxResults) # Return result return res
api_validation_error is interesting: it returns a proper HTTP error status,
and a JSON body with the details of the error, taken straight from the form
validators. It took me a while to find out that the form errors are in
pylons.c.form_errors (and for reference, the form values are in
pylons.c.form_values). pylons.response is a WebOb Response that we can play with.
So now our client side is able to call the API methods, and get a proper error if it calls them wrong.
But now that we have the forms ready, it doesn't take much to display them in web pages as well:
def _describe(self, method): "Return a dict describing an API method" ldesc = getattr(self.engine, method).__doc__.strip() sdesc = ldesc.split("\n")[0] return dict(name=method, sdesc = sdesc, ldesc = ldesc) @expose("myappserver.templates.myappapi") def index(self): ''' Show an index of exported API methods ''' methods = dict() for m in forms.keys(): methods[m] = self._describe(m) return dict(methods=methods) @expose('myappserver.templates.testform') def testform(self, method, **kw): ''' Show a form with the parameters of an API method ''' kw["method"] = method return dict(method=method, action="/myapp/test/"+method, value=kw, info=self._describe(method), form=forms[method]) @expose(content_type="text/plain") @validate(validator_dispatcher, error_handler=testform) def test(self, method, **kw): ''' Run an API method and show its prettyprinted result ''' res = getattr(self, str(method))(**kw) return pprint.pformat(res)
In a few lines, we have all we need: an index of the API methods (including their documentation taken from the docstrings!), and for each method a form to invoke it and a page to see the results.
Make the forms children of AjaxForm, and you can even see the results together with the form.
It is possible to create process pipelines using subprocess.Popen, by just
using stdout=subprocess.PIPE and stdin=otherproc.stdout.
Almost.
In a pipeline created in this way, the stdout of all processes except the last is opened twice: once in the script that has run the subprocess and another time in the standard input of the next process in the pipeline.
This is a problem because if a process closes its stdin, the previous process
in the pipeline does not get SIGPIPE when trying to write to its stdout,
because that pipe is still open on the caller process. If this happens, a wait
on that process will hang forever: the child process waits for the parent to
read its stdout, the parent process waits for the child process to exit.
The trick is to close the stdout of each process in the pipeline except the last just after creating them:
#!/usr/bin/python # coding=utf-8 import subprocess def pipe(*args): ''' Takes as parameters several dicts, each with the same parameters passed to popen. Runs the various processes in a pipeline, connecting the stdout of every process except the last with the stdin of the next process. ''' if len(args) < 2: raise ValueError, "pipe needs at least 2 processes" # Set stdout=PIPE in every subprocess except the last for i in args[:-1]: i["stdout"] = subprocess.PIPE # Runs all subprocesses connecting stdins and stdouts to create the # pipeline. Closes stdouts to avoid deadlocks. popens = [subprocess.Popen(**args[0])] for i in range(1,len(args)): args[i]["stdin"] = popens[i-1].stdout popens.append(subprocess.Popen(**args[i])) popens[i-1].stdout.close() # Returns the array of subprocesses just created return popens
At this point, it's nice to write a function that waits for the whole pipeline to terminate and returns an array of result codes:
def pipe_wait(popens): ''' Given an array of Popen objects returned by the pipe method, wait for all processes to terminate and return the array with their return values. ''' results = [0] * len(popens) while popens: last = popens.pop(-1) results[len(popens)] = last.wait() return results
And, look and behold, we can now easily run a pipeline and get the return codes of every single process in it:
process1 = dict(args='sleep 1; grep line2 testfile', shell=True) process2 = dict(args='awk \'{print $3}\'', shell=True) process3 = dict(args='true', shell=True) popens = pipe(process1, process2, process3) result = pipe_wait(popens) print result
Update: Colin Watson suggests an improvement to compensate for Python's nonstandard SIGPIPE handling.
Colin Watson has a similar library for C.
Suppose you have a User that can be a member of a Company. In SQLObject you
model it somehow like this:
class Company(SQLObject): name = UnicodeCol(length=16, alternateID=True, alternateMethodName="by_name") display_name = UnicodeCol(length=255) class User(InheritableSQLObject): company = ForeignKey("Company", notNull=False, cascade='null')
Then you want to make a form that allows to choose what is the company of a user:
def companies(): return [ [ -1, 'None' ] ] + [ [c.id, c.display_name] for c in Company.select() ] class NewUserFields(WidgetsList): """Fields for editing general settings""" user_name = TextField(label="User name") companyID = SingleSelectField(label="Company", options=companies)
Ok. Now you want to run tests:
nosetestsimports the controller to see if there's any initialisation code.- The NewUserFields class is created.
- The SingleSelectField is created.
- The SingleSelectField constructor tries to guess the validator and peeks at the first option.
- This calls
companies. companiesaccesses the database.- The testing database has not yet been created because nosetests imported the module before giving the test code a chance to setup the test database.
- Bang.
The solution is to add an explicit validator to disable this guessing code that is a source of so many troubles:
class NewUserFields(WidgetsList): """Fields for editing general settings""" user_name = TextField(label="User name") companyID = SingleSelectField(label="Company", options=companies, validator=v.Int(not_empty=True))
I had a great idea:
@validate(model_form)
@error_handler()
@expose(template='kid:myproject.templates.new')
def new(self, id, tg_errors=None, **kw):
"""Create new records in model"""
if tg_errors:
# Ask until there is still something missing
return dict(record = defaults, form = model_form)
else:
# We have everything: save it
i = Item(**kw)
flash("Item was successfully created.")
raise redirect("../show/%d" % i.id)
It was perfect: one simple method, simple error handling, nice helpful messages all around. Except, check boxes and select fields would not get the default values while all other fields would.
After two hours searching and cursing and tracing things into widget code, I
found this bit in InputWidget.adjust_value:
# there are some input fields that when nothing is checked/selected # instead of sending a nice name="" are totally missing from # input_values, this little workaround let's us manage them nicely # without interfering with other types of fields, we need this to # keep track of their empty status otherwise if the form is going to be # redisplayed for some errors they end up to use their defaults values # instead of being empty since FE doesn't validate a failing Schema. # posterity note: this is also why we need if_missing=None in # validators.Schema, see ticket #696.
So, what is happening here is that since check boxes and option fields don't have a nice behaviour when unselected, turbogears has to work around it. So in order to detect the difference between "I selected 'None'" and "I didn't select anything", it reasons that if the input has been validated, then the user has made some selections, so it defaults to "The user selected 'None'". If the input has not been validated, then we're showing the form for the first time, then a missing value means "Use the default provided".
Since I was doing the validation all the time, this meant that Checkboxes and Select fields would never use the default values.
Hence, if you use those fields then you necessarily need two different controller methods, one to present the form and one to save it:
@expose(template='kid:myproject.templates.new')
def new(self, id, **kw):
"""Create new records in model"""
return dict(record = defaults(), form = model_form)
@validate(model_form)
@error_handler(new)
@expose()
def savenew(self, id, **kw):
"""Create new records in model"""
i = Item(**kw)
flash("Item was successfully created.")
raise redirect("../show/%d"%i.id)
If someone else stumbles on the same problem, I hope they'll find this post and they won't have to spend another two awful hours tracking it down again.