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18.22.2 Calling the Deferred
Now you can use the after_this_request decorator to mark a function to be calledat the
endof therequest. But westillneed tocallthem. Forthis thefollowingfunction needs
to beregisteredas after_request() callback:
def call_after_request_callbacks(response):
for callback in getattr(g, after_request_callbacks, ()):
return response
18.22.3 A Practical Example
Now wecan easily atany pointin timeregister a function tobecalledat theendof this
particular request. For exampleyoucan remember thecurrent language of the user in
acookie in the before-request function:
from flask import request
def detect_user_language():
language = request.cookies.get(user_lang)
if language is None:
language = guess_language_from_request()
def remember_language(response):
response.set_cookie(user_lang, language)
g.language = language
18.23 Adding HTTP Method Overrides
SomeHTTPproxiesdonot supportarbitraryHTTPmethodsor newerHTTPmethods
(such asPATCH). In that caseit’spossibleto“proxy” HTTPmethods through another
HTTPmethod in total violation of theprotocol.
The way this works is by letting the client do an HTTP POST request and set the
X-HTTP-Method-Override headerandset thevaluetotheintendedHTTPmethod(such
as PATCH).
Thiscan easily be accomplished with an HTTP middleware:
class HTTPMethodOverrideMiddleware(object):
allowed_methods frozenset([
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bodyless_methods frozenset([GETHEADOPTIONSDELETE])
def __init__(self, app):
self.app app
def __call__(self, environ, , start_response):
method = environ.get(HTTP_X_HTTP_METHOD_OVERRIDE).upper()
if method in self.allowed_methods:
method = method.encode(asciireplace)
environ[REQUEST_METHOD= method
if method in self.bodyless_methods:
return self.app(environ, start_response)
To use this with Flask this is all that is necessary:
from flask import Flask
app = Flask(__name__)
app.wsgi_app = HTTPMethodOverrideMiddleware(app.wsgi_app)
18.24 Request Content Checksums
Various pieces of code can consume the request data and preprocess it. For instance
JSON data ends up on the request object already read and processed, form data ends
upthereaswellbut goesthrougha differentcodepath. Thisseemsinconvenientwhen
you want to calculate the checksum of the incoming request data. This is necessary
sometimesfor some APIs.
Fortunately this ishowever very simpletochangeby wrapping the input stream.
The following example calculates the SHA1 checksum of the incoming data as it gets
readandstoresit in theWSGI environment:
import hashlib
class ChecksumCalcStream(object):
def __init__(self, stream):
self._stream stream
self._hash hashlib.sha1()
def read(self, bytes):
rv self._stream.read(bytes)
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return rv
def readline(self, size_hint):
rv self._stream.readline(size_hint)
return rv
def generate_checksum(request):
env = request.environ
stream = ChecksumCalcStream(env[wsgi.input])
env[wsgi.input= stream
return stream._hash
To use this, all you need to do is to hook the calculating stream in before the request
starts consuming data. (Eg: be careful accessing request.form or anything of that
nature. before_request_handlers for instance should becarefulnot toaccess it).
Example usage:
@app.route(/special-api, methods=[POST])
def special_api():
hash generate_checksum(request)
# Accessing this parses the input stream
files = request.files
# At this point the hash is fully constructed.
checksum hash.hexdigest()
return Hash was: %s  checksum
18.25 Celery Based Background Tasks
Celeryis a taskqueue for Python with batteriesincluded. It used to have a Flask inte-
gration but it became unnecessary after some restructuring of the internals of Celery
with Version 3. This guidefills in the blanksin how to properly use Celery with Flask
but assumes that you generally already read theFirstStepswithCelery guide in the
official Celery documentation.
18.25.1 Installing Celery
Celery is on the Python Package Index (PyPI), so it can be installed with standard
Python tools like pip or easy_install:
$ pip install celery
18.25.2 Configuring Celery
The first thing you need is a Celery instance, this is called the celery application. It
serves thesamepurposeastheFlask objectin Flask,just for Celery. Since thisinstance
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is used as the entry-point for everything you want to do in Celery, like creating tasks
and managing workers, it must be possible for other modules to import it.
For instance you can placethis in a tasks module. While you can use Celery without
any reconfiguration with Flask, it becomes a bit nicer bysubclassingtasks andadding
support for Flask’s application contexts and hooking it up with the Flask configura-
Thisis all that is necessary toproperlyintegrateCelery with Flask:
from celery import Celery
def make_celery(app):
celery = Celery(app.import_name, broker=app.config[CELERY_BROKER_URL])
TaskBase = celery.Task
class ContextTask(TaskBase):
abstract True
def __call__(self*args, **kwargs):
with app.app_context():
return TaskBase.__call__(self*args, **kwargs)
celery.Task = ContextTask
return celery
The function creates a new Celery object, configures it with the broker from the ap-
plication config, updates the rest of the Celery config from the Flask config and then
createsa subclass of the taskthat wraps the task execution in an application context.
18.25.3 Minimal Example
With what we have above this is theminimalexampleof using Celery with Flask:
from flask import Flask
app = Flask(__name__)
celery = make_celery(app)
def add_together(a, b):
return b
Thistask can now becalled in the background:
>>> result add_together.delay(2342)
>>> result.wait()
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18.25.4 Running the Celery Worker
Now if you jumped in and already executed the above codeyouwill bedisappointed
to learn that your .wait() will never actually return. That’s because you also need to
run celery. You can do that by running celery as a worker:
$ celery -A your_application worker
Theyour_application stringhastopoint toyourapplication’spackageormodulethat
createsthecelery object.
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Depending on what you have available there are multiple ways to run Flask applica-
tions. You can use the builtin server during development, but you should use a full
deployment option for production applications. (Do not use the builtin development
server in production.) Several options areavailable and documented here.
If you have a different WSGI server look up the server documentation about how to
use a WSGI appwith it. Just remember that your Flask application object istheactual
WSGI application.
For hosted options to get up and running quickly, see Deploying to a Web Server in the
19.1 mod_wsgi (Apache)
If you areusing theApachewebserver, consider usingmod_wsgi.
Watch Out
Pleasemakesurein advancethat anyapp.run() calls you might havein your applica-
tion file are inside an if __name__ == ’__main__’: block or moved to a separate file.
Just make sure it’s not called becausethis willalways start a local WSGI server which
we donot want if we deploy that application to mod_wsgi.
19.1.1 Installing mod_wsgi
If you don’t have mod_wsgi installed yet you have to either install it using a package
manager or compile it yourself. The mod_wsgiinstallationinstructions cover source
installations on UNIXsystems.
If you areusing Ubuntu/Debian you can apt-get it and activateit as follows:
# apt-get install libapache2-mod-wsgi
On FreeBSD install mod_wsgi by compiling the www/mod_wsgi port or by using
# pkg_add -r mod_wsgi
If youare usingpkgsrc youcan installmod_wsgiby compiling the www/ap2-wsgipack-
If youencountersegfaultingchildprocessesafter thefirst apachereloadyoucan safely
ignore them. Just restart theserver.
19.1.2 Creating a .wsgi file
To run your application you need ayourapplication.wsgifile. Thisfilecontains the code
mod_wsgi is executing on startup to get the application object. The object called appli-
cation in that file is then used as application.
For most applications the following file shouldbe sufficient:
from yourapplication import app as application
If you don’t have a factory function for application creation but a singleton instance
youcan directly import that one as application.
Store that file somewhere that you will find it again (e.g.: /var/www/yourapplication)
andmakesurethat yourapplicationandallthelibraries that arein useareonthepython
load path. If you don’t want to install it system wide consider using avirtualpython
instance. Keep in mind that youwill have to actually installyour application into the
virtualenv as well. Alternatively there is the option to just patch the path in the .wsgi
file before the import:
import sys
19.1.3 Configuring Apache
The last thing you have to do is to create an Apache configuration file for your ap-
plication. In this example we are telling mod_wsgi to execute the application under a
different user for security reasons:
<VirtualHost *>
ServerName example.com
WSGIDaemonProcess yourapplication user=user1 group=group1 threads=5
WSGIScriptAlias /var/www/yourapplication/yourapplication.wsgi
<Directory /var/www/yourapplication>
WSGIProcessGroup yourapplication
WSGIApplicationGroup %{GLOBAL}
Order deny,allow
Allow from all
Note: WSGIDaemonProcess isn’t implementedin Windowsand Apachewillrefuseto
run with the above configuration. On a Windows system, eliminate those lines:
<VirtualHost *>
ServerName example.com
WSGIScriptAlias / C:\yourdir\yourapp.wsgi
<Directory C:\yourdir>
Order deny,allow
Allow from all
For moreinformation consult themod_wsgiwiki.
19.1.4 Troubleshooting
If your application does not run, follow this guide to troubleshoot:
Problem: applicationdoes not run, errorlog shows SystemExit ignored You have a
app.run() call in your application file that is not guarded by an if __name__
== ’__main__’: condition. Either remove that run() call from the file and move
it into a separate run.pyfileor put it intosuch an if block.
Problem: applicationgives permission errors Probably caused by your application
running asthewrong user. Makesure the folderstheapplication needs access to
have the proper privileges set and the application runs as the correct user (user
and group parameter totheWSGIDaemonProcess directive)
Problem: applicationdies with an error on print Keep in mind that mod_wsgi dis-
allows doing anything withsys.stdout andsys.stderr. You can disable this
protection from theconfig bysetting the WSGIRestrictStdout to off:
WSGIRestrictStdout Off
Alternativelyyoucan also replacethe standardout in the .wsgi filewith a differ-
ent stream:
import sys
sys.stdout = sys.stderr
Problem: accessing resources gives IO errors Your application probably is a single
.py file you symlinked into the site-packages folder. Please be aware that this
does not work, instead youeither have toput the folder into the pythonpath the
file isstoredin, or convert your application into a package.
The reason for this is that for non-installed packages, the module filename is
used to locate the resources and for symlinksthewrongfilenameis pickedup.
19.1.5 Support for Automatic Reloading
Tohelpdeploymenttoolsyoucan activatesupport forautomatic reloading. Whenever
somethingchangesthe.wsgifile,mod_wsgiwillreloadallthedaemon processesfor us.
For that, just add thefollowing directive to your Directory section:
WSGIScriptReloading On
19.1.6 Working with Virtual Environments
Virtual environments have the advantage that they never install the required depen-
dencies system wide so you have a better control over what is used where. If you
want to usea virtual environment with mod_wsgi you have to modify your .wsgi file
Add the following lines to the topof your .wsgifile:
activate_this /path/to/env/bin/activate_this.py
execfile(activate_this, dict(__file__=activate_this))
This sets upthe load paths according tothe settings of the virtual environment. Keep
in mind that thepath hastobe absolute.
19.2 Standalone WSGI Containers
Therearepopular servers written in Python thatcontain WSGI applications andserve
HTTP. These servers stand alone when they run; you can proxy to them from your
web server. Note the section on Proxy Setups if yourun into issues.
19.2.1 Gunicorn
model ported from Ruby’s Unicorn project. It supports botheventletandgreenlet.
Running a Flask application on this server is quite simple:
gunicorn myproject:app
Gunicornprovidesmanycommand-lineoptions–seegunicorn -h. . Forexample,to
run a Flask application with 4 worker processes (-w 4) binding to localhost port 4000
gunicorn -w 4 -b myproject:app
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