Part I
USER’S GUIDE
Thispart of the documentation,which ismostlyprose,beginswith some background
information about Flask, then focuses on step-by-step instructions for web develop-
mentwith Flask.
1
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CHAPTER
ONE
FOREWORD
Read this before you get started with Flask. This hopefully answers some questions
about the purpose and goals of the project, and when you should or should not be
usingit.
1.1 What does “micro” mean?
“Micro”doesnotmeanthatyourwholewebapplicationhastofitintoasingle Python
file, although it certainly can. Nor does itmean thatFlaskislackingin functionality.
The “micro” in microframeworkmeansFlaskaimstokeep the core simplebutexten-
sible. Flask won’t makemanydecisionsfor you,suchaswhatdatabase touse. Those
decisionsthatitdoesmake,suchaswhattemplatingenginetouse,areeasytochange.
Everything else is up to you, so that Flask can be everything you need and nothing
youdon’t.
Bydefault,Flaskdoesnotincludeadatabaseabstractionlayer,formvalidationorany-
thing else where different libraries already exist that can handle that. Instead, Flask
supports extensions to add such functionality toyour application as if it was imple-
mentedin Flaskitself. Numerous extensions provide database integration, form val-
idation, upload handling, variousopen authentication technologies,and more. Flask
maybe“micro”,butit’sreadyforproductionuseonavarietyofneeds.
1.2 Configuration and Conventions
Flask has many configuration values, with sensible defaults, and a few conventions
when gettingstarted. By convention templatesand static filesare stored in subdirec-
tories withinthe application’sPython source tree,withthe namestemplatesandstatic
respectively. While this can be changed you usually don’t have to, especially when
gettingstarted.
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1.3 Growing with Flask
Once you have Flask up and running, you’ll find a variety of extensions available in
the communitytointegrate your project for production. The Flask core team reviews
extensionsandensuresapprovedextensionsdonot breakwithfuture releases.
As your codebase grows, you are free to make the design decisions appropriate for
your project. Flask will continue toprovide a very simple glue layer to the best that
Python has to offer. You can implement advanced patterns in SQLAlchemy or an-
otherdatabasetool,introducenon-relational datapersistenceasappropriate,andtake
advantage of framework-agnostictoolsbuiltforWSGI,the Pythonwebinterface.
Flask includes many hooks to customize its behavior. Should you need more cus-
tomization, the Flaskclass is built for subclassing. Ifyou are interestedin that, check
outtheBecomingBigchapter. IfyouarecuriousabouttheFlaskdesignprinciples,head
over tothe sectionaboutDesignDecisionsinFlask.
Continue toInstallation,the Quickstart,ortheForewordforExperiencedProgrammers.
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CHAPTER
TWO
FOREWORD FOR EXPERIENCED
PROGRAMMERS
2.1 Thread-Locals in Flask
One of the design decisions in Flask was that simple tasks should be simple; they
shouldnottake alot of code andyettheyshouldnotlimit you. Because of that, Flask
hasfewdesignchoicesthatsome peoplemightfindsurprisingorunorthodox. Forex-
ample,Flaskusesthread-local objectsinternallysothatyou don’thave topassobjects
around from function to function within a request in order to stay threadsafe. This
approachisconvenient,butrequiresavalidrequestcontextfor dependency injection
orwhenattemptingtoreusecode whichusesavaluepeggedtothe request. The Flask
projectishonestaboutthread-locals,doesnothidethem,andcallsoutinthecode and
documentationwhere theyareused.
2.2 Develop for the Web with Caution
Alwayskeep securityin mindwhen buildingwebapplications.
If you write awebapplication,you are probablyallowing usersto register andleave
theirdataonyourserver. The usersareentrustingyou withdata. Andevenifyouare
the only userthat might leave data in your application, you still wantthatdata tobe
storedsecurely.
Unfortunately, there are many ways the security of a web application can be com-
promised. Flask protects you against one of the most common security problems of
modern webapplications: cross-site scripting(XSS).Unlessyoudeliberatelymarkin-
secure HTML as secure, Flask and the underlying Jinja2 template engine have you
covered. Butthere are manymorewaystocause securityproblems.
The documentationwill warnyou aboutaspectsof webdevelopmentthatrequire at-
tention to security. Some of these security concerns are far more complex than one
might think, and we all sometimes underestimate the likelihood that a vulnerability
willbe exploited-untilacleverattacker figuresout awaytoexploitour applications.
Anddon’t thinkthat your application is not important enough toattract an attacker.
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Dependingonthekindofattack,chancesarethatautomatedbotsareprobingforways
tofill yourdatabasewith spam,linkstomalicioussoftware,andthe like.
Flask is nodifferent from any other frameworkin that you the developer must build
withcaution,watchingfor exploitswhen buildingtoyourrequirements.
2.3 The Status of Python 3
Currently the Python community is in the process of improving libraries to support
the newiterationofthe Pythonprogramminglanguage. While thesituationisgreatly
improving there are still some issues that make it hard for users to switch over to
Python 3 just now. These problems are partially caused by changesin the language
that went unreviewedfor too long, partially alsobecause we have not quite worked
out howthelower-level APIshouldchangetoaccount fortheUnicode differencesin
Python3.
We strongly recommendusing Python 2.6 and2.7 with activated Python 3warnings
during development. If you plan on upgrading to Python 3 in the near future we
stronglyrecommendthatyoureadHowtowriteforwardscompatiblePythoncode.
IfyoudowanttodiveintoPython3alreadyhavealookatthe python3_supportpage.
Continue toInstallationorthe Quickstart.
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CHAPTER
THREE
INSTALLATION
Flask dependson twoexternal libraries,WerkzeugandJinja2. Werkzeugis a toolkit
for WSGI, the standard Python interface between web applications and a variety of
serversfor both developmentanddeployment. Jinja2renderstemplates.
Sohowdoyougetallthatonyourcomputerquickly? Therearemanywaysyoucould
dothat,but the mostkick-assmethodisvirtualenv,solet’shavealookatthatfirst.
You will need Python 2.6 or higher to get started, so be sure to have an up-to-date
Python2.xinstallation. ForusingFlaskwithPython 3havealookatPython3Support.
3.1 virtualenv
Virtualenv is probably what you want to use during development, and if you have
shell accesstoyour productionmachines,you’ll probablywanttouse itthere,too.
What problemdoesvirtualenvsolve? Ifyou like PythonasmuchasI do, chancesare
you want to use it for other projects besides Flask-based web applications. But the
more projects you have, the more likely it isthat you will be working with different
versionsofPythonitself,orat leastdifferentversionsofPythonlibraries. Let’sface it:
quite often librariesbreakbackwardscompatibility, andit’sunlikely that anyserious
application will have zero dependencies. So what doyou do if two or more of your
projectshave conflictingdependencies?
Virtualenv to the rescue! Virtualenv enables multiple side-by-side installations of
Python,one foreach project. It doesn’t actuallyinstall separate copiesof Python,but
itdoesprovideaclever waytokeep different project environmentsisolated. Let’ssee
howvirtualenvworks.
Ifyouare onMacOSXorLinux,chancesarethatone ofthefollowingtwocommands
willworkfor you:
$ sudo easy_install virtualenv
oreven better:
$ sudo pip install virtualenv
7
One of thesewill probablyinstall virtualenvonyour system. Maybe it’seveninyour
package manager. Ifyou use Ubuntu,try:
$ sudo apt-get install python-virtualenv
If you are on Windows and don’t have the easy_install command, you must install it
first. Check the pip anddistribute onWindowssection for more information abouthow
todothat. Once you have itinstalled,run the same commandsasabove,but without
the sudoprefix.
Once you have virtualenvinstalled,justfire up a shell and create your own environ-
ment. Iusuallycreate aprojectfolderandavenvfolderwithin:
$ mkdir myproject
$ cd myproject
$ virtualenv venv
New python executable in venv/bin/python
Installing distribute............done.
Now, whenever you want to work on a project, you only have to activate the corre-
spondingenvironment.OnOSX andLinux,dothe following:
$ . venv/bin/activate
If you areaWindowsuser,the followingcommandisfor you:
$ venv\scripts\activate
Either way,youshouldnowbeusingyourvirtualenv(notice howthepromptofyour
shell haschangedtoshow the activeenvironment).
Now you can just enter the following command to get Flask activated in your vir-
tualenv:
$ pip install Flask
Afew secondslaterandyouare goodtogo.
3.2 System-Wide Installation
Thisispossibleaswell,thoughIdonotrecommendit.Justrunpipwithrootprivileges:
$ sudo pip install Flask
(OnWindowssystems,runitinacommand-promptwindowwithadministratorpriv-
ileges,andleave outsudo.)
8
3.3 Living on the Edge
IfyouwanttoworkwiththelatestversionofFlask,therearetwoways: youcaneither
let pippull inthe developmentversion, or you can tell ittooperate onagitcheckout.
Either way,virtualenvisrecommended.
Getthe gitcheckoutinanewvirtualenvandrun indevelopment mode:
$ git clone http://github.com/mitsuhiko/flask.git
Initialized empty Git repository in ~/dev/flask/.git/
$ cd flask
$ virtualenv venv --distribute
New python executable in venv/bin/python
Installing distribute............done.
$ . venv/bin/activate
$ python setup.py develop
...
Finished processing dependencies for Flask
This will pull in the dependencies and activate the git head as the current version
inside the virtualenv. Thenall youhave todoisrungit pull origintoupdate tothe
latestversion.
Tojustgetthe developmentversionwithoutgit,dothisinstead:
$ mkdir flask
$ cd flask
$ virtualenv venv --distribute
$ . venv/bin/activate
New python executable in venv/bin/python
Installing distribute............done.
$ pip install Flask==dev
...
Finished processing dependencies for Flask==dev
3.4 pip and distribute on Windows
On Windows, installation of easy_install is a little bit trickier, but still quite easy. The
easiest waytodoitistodownloadthedistribute_setup.pyfile andrun it. The easiest
waytorunthefile istoopenyourdownloadsfolderanddouble-clickon the file.
Next, addthe easy_install commandandotherPython scripts tothe commandsearch
path, by adding your Python installation’s Scripts folder to the PATH environment
variable. To do that, right-click on the “Computer” icon on the Desktop or in the
Start menu, and choose “Properties”. Then click on “AdvancedSystem settings”(in
WindowsXP, clickon the “Advanced”tab instead). Then clickon the “Environment
variables” button. Finally, double-click on the “Path” variable in the “System vari-
ables”section,andaddthepathofyourPythoninterpreter’sScriptsfolder. Be sure to
9
delimit itfrom existingvalueswitha semicolon. Assumingyou are usingPython 2.7
onthe defaultpath,addthe followingvalue:
;C:\Python27\Scripts
And you are done! To check that it worked, open the Command Prompt and exe-
cute easy_install. If you have User Account Control enabled on WindowsVista or
Windows7,itshouldpromptyoufor administratorprivileges.
Nowthatyou have easy_install,you canuse ittoinstall pip:
> easy_install pip
10
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