Pycharm v3.0.1 serial key or number
Pycharm v3.0.1 serial key or number
Welcome to the uncertainties package¶
The uncertainties package is a free, cross-platform program that transparently handles calculations with numbers with uncertainties (like 3.14±0.01). It can also yield the derivatives of any expression.
The package takes the pain and complexity out of uncertainty calculations. Error propagation is not to be feared anymore!
Calculations of results with uncertainties, or of derivatives, can be performed either in an interactive session (as with a calculator), or in programs written in the Python programming language. Existing calculation code can run with little or no change.
Whatever the complexity of a calculation, this package returns its result with an uncertainty as predicted by linear error propagation theory. It automatically calculates derivatives and uses them for calculating uncertainties. Almost all uncertainty calculations are performed analytically.
Correlations between variables are automatically handled, which sets this module apart from many existing error propagation codes.
You may want to check the following related uncertainty calculation Python packages to see if they better suit your needs: soerp (higher-order approximations) and mcerp (Monte-Carlo approach).
An easy-to-use calculator¶
Calculations involving numbers with uncertainties can be performed even without knowing anything about the Python programming language. After installing this package and invoking the Python interpreter, calculations with automatic error propagation can be performed transparently (i.e., through the usual syntax for mathematical formulas):
Thus, existing calculation code designed for regular numbers can run with numbers with uncertainties with no or little modification.
Another strength of this package is its correct handling of correlations. For instance, the following quantity is exactly zero even though has an uncertainty:
Many other error propagation codes return the incorrect value 0±0.1414… because they wrongly assume that the two subtracted quantities are independent random variables.
Arrays of numbers with uncertainties are transparently handled too.
Derivatives are similarly very easy to obtain:
They are calculated with a fast method.
Available documentation¶
The User Guide details many of the features of this package.
The part Uncertainties in arrays describes how arrays of numbers with uncertainties can be created and used.
The Technical Guide gives advanced technical details.
A of the documentation is also available.
Additional information is available through the pydoc command, which gives access to many of the documentation strings included in the code.
Installation and download¶
Important note¶
The installation commands below should be run in a DOS or Unix command shell (not in a Python shell).
Under Windows (version 7 and earlier), a command shell can be obtained by running (through the Run… menu item from the Start menu). Under Unix (Linux, Mac OS X,…), a Unix shell is available when opening a terminal (in Mac OS X, the Terminal program is found in the Utilities folder, which can be accessed through the Go menu in the Finder).
Automatic install or upgrade¶
One of the automatic installation or upgrade procedures below might work on your system, if you have a Python package installer or use certain Linux distributions.
Under Unix, it may be necessary to prefix the commands below with , so that the installation program has sufficient access rights to the system.
If you have pip, you can try to install the latest version with
If you have setuptools, you can try to automatically install or upgrade this package with
The package is also available for Windows through the Python(x,y) distribution. It may also be included in Christoph Gohlke’s Base distribution of scientific Python packages.
Mac OS X users who use the MacPorts package manager can install with , and upgrade it with where represents the desired Python version (, , etc.).
The package is also available through the following Linux distributions and software platforms: Ubuntu, Fedora, openSUSE, Debian and Maemo.
Manual download and install¶
Alternatively, you can simply download the package archive from the Python Package Index (PyPI) and unpack it. The package can then be installed by going into the unpacked directory (), and running the provided program with
(where the default interpreter must generally be replaced by the version of Python for which the package should be installed: , , etc.).
For an installation with Python 2.6+ in the user Python library (no additional access rights needed):
For an installation in a custom directory :
If additional access rights are needed (Unix):
You can also simply move the directory that corresponds best to your version of Python to a location that Python can import from (directory in which scripts using are run, etc.); the chosen directory should then be renamed . Python 3 users should then run from inside this directory so as to automatically adapt the code to Python 3.
Source code¶
The latest, bleeding-edge but working code and documentation source are available on GitHub. The package is written in pure Python and has no external dependency (the NumPy package is optional). It contains about 7000 lines of code. 75 % of these lines are documentation strings and comments. The remaining 25 % are split between unit tests (15 % of the total) and the calculation code proper (10 % of the total). is thus a lightweight, portable package with abundant documentation and tests.
Migration from version 1 to version 2¶
Some incompatible changes were introduced in version 2 of (see the version history). While the version 2 line will support the version 1 syntax for some time, it is recommended to update existing programs as soon as possible. This can be made easier through the provided automatic updater.
The automatic updater works like Python’s 2to3 updater. It can be run (in a Unix or DOS shell) with:
For example, updating a single Python program can be done with
All the Python programs contained under a directory (including in nested sub-directories) can be automatically updated with
Backups are automatically created, unless the option is given.
Some manual adjustments might be necessary after running the updater (incorrectly modified lines, untouched obsolete syntax).
While the updater creates backup copies by default, it is generally useful to first create a backup of the modified directory, or alternatively to use some version control system. Reviewing the modifications with a file comparison tool might also be useful.
What others say¶
- “Superb,” “wonderful,” “It’s like magic.” (Joaquin Abian)
- “pretty amazing” (John Kitchin)
- “An awesome python package” (Jason Moore)
- “Utterly brilliant” (Jeffrey Simpson)
- “An amazing time saver” (Paul Nakroshis)
- “Seems to be the gold standard for this kind of thing” (Peter Williams)
- “This package has a great interface and makes error propagation something to stop fearing.” (Dr Dawes)
- “uncertainties makes error propagation dead simple.” (enrico documentation)
- “many inspiring ideas” (Abraham Lee)
- “Those of us working with experimental data or simulation results will appreciate this.” (Konrad Hinsen)
- “PyPI’s uncertainties rocks!” (Siegfried Gevatter)
- “A very cool Python module” (Ram Rachum)
- “Holy f*** this would have saved me so much f***ing time last semester.” (reddit)
Future developments¶
Planned future developments include (starting from the most requested ones):
- handling of complex numbers with uncertainties;
- increased support for NumPy: Fourier Transform with uncertainties, automatic wrapping of functions that accept or produce arrays, standard deviation of arrays, more convenient matrix creation, new linear algebra methods (eigenvalue and QR decompositions, determinant,…), input of arrays with uncertainties as strings (like in NumPy),…;
- JSON support;
- addition of and attributes, for increased compatibility with existing code (Python numbers have these attributes);
- addition of new functions from the module;
- fitting routines that conveniently handle data with uncertainties;
- a re-correlate function that puts correlations back between data that was saved in separate files;
- support for multi-precision numbers with uncertainties.
Call for contributions: I got multiple requests for complex numbers with uncertainties, Fourier Transform support, and the automatic wrapping of functions that accept or produce arrays. Please contact me if you are interested in contributing. Patches are welcome. They must have a high standard of legibility and quality in order to be accepted (otherwise it is always possible to create a new Python package by branching off this one, and I would still be happy to help with the effort).
Please support the continued development of this program by donating $10 or more through PayPal (no PayPal account necessary). I love modern board games, so this will go towards giving my friends and I some special gaming time!
How to cite this package¶
If you use this package for a publication (in a journal, on the web, etc.), please cite it by including as much information as possible from the following: Uncertainties: a Python package for calculations with uncertainties, Eric O. LEBIGOT, http://pythonhosted.org/uncertainties/. Adding the version number is optional.
Acknowledgments¶
The author wishes to thank all the people who made generous donations: they help keep this project alive by providing positive feedback.
I greatly appreciate having gotten key technical input from Arnaud Delobelle, Pierre Cladé, and Sebastian Walter. Patches by Pierre Cladé, Tim Head, José Sabater Montes, Martijn Pieters, Ram Rachum, Christoph Deil, Gabi Davar and Roman Yurchak are gratefully acknowledged.
I would also like to thank users who contributed with feedback and suggestions, which greatly helped improve this program: Joaquin Abian, Jason Moore, Martin Lutz, Víctor Terrón, Matt Newville, Matthew Peel, Don Peterson, Mika Pflueger, Albert Puig, Abraham Lee, Arian Sanusi, Martin Laloux, Jonathan Whitmore, Federico Vaggi, Marco A. Ferra, Hernan Grecco, David Zwicker, James Hester, Andrew Nelson, and many others.
I am also grateful to Gabi Davar and Pierre Raybaut for including it in Python(x,y), to Christoph Gohlke for including it in his Base distribution of scientific Python packages for Windows, and to the Anaconda, macOS and Linux distribution maintainers of this package (Jonathan Stickel, David Paleino, Federico Ceratto, Roberto Colistete Jr, Filipe Pires Alvarenga Fernandes, and Felix Yan).
License¶
This software is released under a dual license; one of the following options can be chosen:
- The Revised BSD License (© 2010–2016, Eric O. LEBIGOT [EOL]).
- Any other license, as long as it is obtained from the creator of this package.
Cannot connect to a database
If you are completely new to DataGrip, read the Quick start guide that gives a quick overview of the first steps with the IDE.
Step 1. Check your network settings
Databases can work locally, on a server, or in the cloud. For server and cloud databases, you need a network connection. To verify that connection is available, use ping and telnet commands.
With the ping command, you can ensure that the destination computer is reachable from the source computer. Open a command line and type the following command: , where is a command option that resolves addresses to hostnames (if it is possible). If you use hostnames with the ping command, a hostname is resolved to the IP address. For example, resolves to .
With the telnet command, you can test connectivity to remote computers and issue commands. If you specify a port as a parameter for the command, you can test connectivity to a remote host on the given port. If the connection is successful, you see the message: .
For security reasons, DBMS usually drops all telnet connections. The telnet command allows you to check if the port is opened for communication.
Step 2. Check your connection properties
Each database (MySQL, PostgreSQL, Oracle, or any other vendor) has its own connection settings. Most database include the connection settings:
Host: A hostname of a computer or other device that stores a database. It can be an IP address 127.0.0.1 or a domain name localhost.
Database: A name of the database to which you want to connect. You can find the database name in the settings of your database server, or you can ask your database administrator. In some cases, it is possible to run a query in a database command line to see names of all available databases. For example, in MySQL you can run .
User: A name of a user that has sufficient privileges to perform actions with a database. Run a query in a database command line to see names of all available databases. For example, in MySQL you can run .
Password: A password of the user.
Port: An number that identifies a connection point between hosts. Hosts use port numbers to determine to which application, service, or process a connection must be established. Different database vendors use different ports for their databases. The following list is a list of default port numbers.
Vendor Default port Amazon Redshift 5439 Apache Derby 1527 Apache Cassandra 9042 Apache Hive 10000 (Hive Server2) or 9083 (Hive Metastore) Azure SQL Database 1433 ClickHouse 8123 Exasol 8563 Greenplum 5432 H2 8082 HSQLDB 9001 IBM Db2 50000 MariaDB 3306 Microsoft SQL Server 1433 (TCP), 1434 (UDP might be required) MySQL 3306 Oracle 1521 PostgreSQL 5432 Snowflake 443 SQLite None Sybase ASE 5000 Vertica 5433 Real port numbers might be different on your system. Verify that you use a correct port number with your database administrator.
Verify that the connection settings for the selected database connection are correct. For more information about creating or changing a database connection, see Database connection.
Step 3. Check the driver version
With a JDBC driver, you can interact with a database management system (DBMS) from DataGrip. Each DBMS requires its own JDBC driver. Ensure that the driver version and the DBMS version are compatible with each other.
From DataGrip, you can download drivers for all supported vendors. You can check the full list of supported vendors in the Drivers list . Alternatively, you can add your own driver to an existing vendor, or create a new driver entry for the vendor that is not on the Drivers list.
Download a driver and select the driver version
To download drivers from the JetBrains FTP server, select a vendor from the Drivers list, and click the Download ver. <version_number> link in the Driver files pane.
To change the driver version, click the ver. <version_number> link in the Driver files pane and select the driver version that you need.
Create a connection to a database with a JDBC driver
If you cannot find a name of a database vendor in the list of data sources, download a JDBC driver for the database management system (DBMS), and create a connection in DataGrip. With the JDBC driver, you can connect to DBMS and start working.
Navigate to File | Data Sources or press .
In the Data Sources and Drivers dialog, click the Add icon (
) and select Driver and Data Source.Click the User Driver link.
In the Driver files pane, click the Add icon (
) and select Custom JARs.Navigate to the JAR file of the JDBC driver, select it, and click OK.
In the Class field, specify the value that you want to use for the driver.
Click Apply.
Return to the created data source connection.
Specify database connection details. Alternatively, paste the JDBC URL in the URL field.
To delete a password, right-click the Password field and select Set empty.
To ensure that the connection to the data source is successful, click Test Connection.
Add a JDBC driver to an existing connection
Navigate to File | Data Sources or press .
In the Data Sources and Drivers dialog, select a data source where you want to change a driver, and click the Driver link in data source settings.
Click the provided driver entry, and click Remove (
).To revert changes, click Reset that is in the upper-right part of the window. Alternatively, click the Add button (
) and select Provided Driver | <database_name> | <driver_version>In the Driver files pane, click the Add icon (
) and select Custom JARs.Navigate to the JAR file of the JDBC driver, select it, and click OK.
In the Class field, specify the value that you want to use for the driver.
Click Apply.
Step 4. Check if the connection with SSH or SSL is required
To make a connection to a database more secure, some services require SSH or SSL usage.
SSL
The following procedure describes the SSL configuration that suits most databases. For some databases, you need to use another approach for a successful connection. See the Tutorials section that includes configuration examples for Apache Cassandra, Heroku Postgres, and MySQL 5.1.
Connect to a database with SSL
Navigate to File | Data Sources or press .
Select a data source profile where you want to change connection settings.
Click the SSH/SSL tab and select the Use SSL checkbox.
In the CA file field, navigate to the CA certificate file (for example, mssql.pem).
In the Client certificate file field, navigate to the client certificate file (for example, client-cert.pem).
In the Client key file field, navigate to the client key file (for example, client-key.pem).
From the Mode list, select the verification mode:
Require: verifies that the server accepts SSL connections for this IP address and recognizes the client certificate.
Verify CA: verifies the server by checking the certificate chain up to the root certificate that is stored on the client.
Full verification: verifies the server host to ensure that it matches the name stored in the server certificate. The SSL connection fails if the server certificate cannot be verified.
To ensure that the connection to the data source is successful, click Test Connection.
It is recommended to use PEM certificates.
With self-signed certificates and in some cases with certificates issued by the trusted root entity, you might experience errors when you use the latest JDBC driver version. The SSL connection might fail if your Java keystore does not accept the certificate chains. As a temporary solution, try to downgrade the JDBC driver (for example, for the MySQL connector, you need to switch to the 5.1.40 version.)
Copy SSL settings from other data sources
If you configured SSL settings for one data source, you can copy them for another data source.
Navigate to File | Data Sources or press .
Select a data source profile where you want to change connection settings.
Click the SSH/SSL tab and select the Use SSL checkbox.
Click the Copy from link and select the configuration that you want to copy.
SSH
Secure Shell or SSH is a network protocol that is used to encrypt a connection between a client and a server.
All created SSH connections are shared between all the data sources that you have in a project. If you do not want to share a connection between projects, select the Visible only for this project checkbox in the SSH connection settings.
Connect to a database with SSH
Navigate to File | Data Sources or press .
Select a data source profile where you want to change connection settings.
Click the SSH/SSL tab and select the Use SSH tunnel checkbox.
Click the Add SSH configuration button (
).In the SSH dialog, click the Add button.
If you do not want to share the configuration between projects, select the Visible only for this project checkbox.
In Host, User name, and Port fields, specify your connection details.
From the Auth type list, you can select an authentication method:
Password: to access the host with a password. To save the password in DataGrip, select the Save password checkbox.
Key pair (OpenSSH or PuTTY): to use SSH authentication with a key pair. To apply this authentication method, you must have a private key on the client machine and a public key on the remote server. DataGrip supports private keys that are generated with the OpenSSH utility.
Specify the path to the file where your private key is stored and type the passphrase (if any) in the corresponding fields. To have DataGrip remember the passphrase, select the Save passphrase checkbox.
OpenSSH config and authentication agent: to use SSH keys that are managed by a credentials helper application (for example, Pageant on Windows or ssh-agent on macOS and Linux).
To ensure that the connection to the data source is successful, click Test Connection.
See the Generating a new SSH key and adding it to the ssh-agent tutorial for details on working with SSH keys.
Create the SSH tunnel with PuTTY (Windows)
Download and run the latest version of the PuTTY SSH and Telnet client (download the client from https://www.putty.org/).
In the PuTTY Configuration dialog, navigate to Connection | SSH | Auth.
In the Private key file for authentication field, specify the path to your private key file and click Open.
In the command line window, specify the username that you use for the SSH tunnel and press . Do not close the command line window.
In DataGrip, navigate to File | Data Sources.
Select a data source profile where you want to change connection settings.
Click the SSH/SSL tab and select the Use SSH tunnel checkbox.
From the Auth type list, select OpenSSH config and authentication agent.
In Proxy host, Proxy user, and Port fields, specify connection details.
To ensure that the connection to the data source is successful, click Test Connection.
Create the SSH tunnel with Pageant (Windows)
Pageant is an SSH authentication agent for PuTTY, PSCP, PSFTP, and Plink. Pageant stores your private key, and as long as it is running, it provides the unlocked private key to PuTTY or other tools like DataGrip. You can find the Pageant icon in the Windows taskbar.
Download the latest version of Pageant (download the client from https://www.putty.org/).
In the Windows taskbar, right-click the Pageant icon and select Add Key.
In the Select Private Key File dialog, navigate to the private key file (the PPK file) and click Open.
(Optional) Enter the private key passphrase and press .
In DataGrip, navigate to File | Data Sources.
Select a data source profile where you want to change connection settings.
Click the SSH/SSL tab and select the Use SSH tunnel checkbox.
From the Auth type list, select OpenSSH config and authentication agent.
In Proxy host, Proxy user, and Port fields, specify connection details.
To ensure that the connection to the data source is successful, click Test Connection.
Create the SSH tunnel with the ssh-agent (macOS and Linux)
Run all commands for ssh-agent in the command line.
Ensure that ssh-agent is running.
Add your key to the agent (in the following example, the key path is ~/.ssh/id_rsa).
(Optional) On macOS, you can add option to the command to store passphrases in your keychain. On macOS Sierra and later, you need to create the config file in ~/.ssh/ with the following text:
Host * UseKeychain yes AddKeysToAgent yes IdentityFile ~/.ssh/id_rsaIf you have other private keys in the .ssh directory, add an line for each key. For example, if the second key has the id_ed25519 name, add as an additional line for the second private key.
List all added keys.
In DataGrip, navigate to File | Data Sources.
Select a data source profile where you want to change connection settings.
Click the SSH/SSL tab and select the Use SSH tunnel checkbox.
From the Auth type list, select OpenSSH config and authentication agent.
In Proxy host, Proxy user, and Port fields, specify connection details.
To ensure that the connection to the data source is successful, click Test Connection.
Step 5. Write to us if you still need help
Write to the DataGrip team
Email our team at datagrip@jetbrains.com. Describe your problem, and attach all available materials that can speed up troubleshooting (code samples, screenshots, logs, animations, videos, and other materials).
For more information about other troubleshooting sources, see Troubleshooting materials.
Documentation
Let’s learn by example.
Throughout this tutorial, we’ll walk you through the creation of a basic poll application.
It’ll consist of two parts:
- A public site that lets people view polls and vote in them.
- An admin site that lets you add, change and delete polls.
We’ll assume you have Django installed already. You can tell Django is installed and which version by running the following command:
If Django is installed, you should see the version of your installation. If it isn’t, you’ll get an error telling “No module named django”.
This tutorial is written for Django 1.8 and Python 3.2 or later. If the Django version doesn’t match, you can refer to the tutorial for your version of Django by using the version switcher at the bottom right corner of this page, or update Django to the newest version. If you are still using Python 2.7, you will need to adjust the code samples slightly, as described in comments.
See How to install Django for advice on how to remove older versions of Django and install a newer one.
Where to get help:
If you’re having trouble going through this tutorial, please post a message to django-users or drop by #django on irc.freenode.net to chat with other Django users who might be able to help.
Creating a project¶
If this is your first time using Django, you’ll have to take care of some initial setup. Namely, you’ll need to auto-generate some code that establishes a Django project – a collection of settings for an instance of Django, including database configuration, Django-specific options and application-specific settings.
From the command line, into a directory where you’d like to store your code, then run the following command:
This will create a directory in your current directory. If it didn’t work, see Problems running django-admin.
Note
You’ll need to avoid naming projects after built-in Python or Django components. In particular, this means you should avoid using names like (which will conflict with Django itself) or (which conflicts with a built-in Python package).
Where should this code live?
If your background is in plain old PHP (with no use of modern frameworks), you’re probably used to putting code under the Web server’s document root (in a place such as ). With Django, you don’t do that. It’s not a good idea to put any of this Python code within your Web server’s document root, because it risks the possibility that people may be able to view your code over the Web. That’s not good for security.
Put your code in some directory outside of the document root, such as .
Let’s look at what created:
These files are:
- The outer root directory is just a container for your project. Its name doesn’t matter to Django; you can rename it to anything you like.
- : A command-line utility that lets you interact with this Django project in various ways. You can read all the details about in django-admin and manage.py.
- The inner directory is the actual Python package for your project. Its name is the Python package name you’ll need to use to import anything inside it (e.g. ).
- : An empty file that tells Python that this directory should be considered a Python package. (Read more about packages in the official Python docs if you’re a Python beginner.)
- : Settings/configuration for this Django project. Django settings will tell you all about how settings work.
- : The URL declarations for this Django project; a “table of contents” of your Django-powered site. You can read more about URLs in URL dispatcher.
- : An entry-point for WSGI-compatible web servers to serve your project. See How to deploy with WSGI for more details.
Database setup¶
Now, open up . It’s a normal Python module with module-level variables representing Django settings.
By default, the configuration uses SQLite. If you’re new to databases, or you’re just interested in trying Django, this is the easiest choice. SQLite is included in Python, so you won’t need to install anything else to support your database. When starting your first real project, however, you may want to use a more robust database like PostgreSQL, to avoid database-switching headaches down the road.
If you wish to use another database, install the appropriate database bindings, and change the following keys in the item to match your database connection settings:
- – Either , , , or . Other backends are also available.
- – The name of your database. If you’re using SQLite, the database will be a file on your computer; in that case, should be the full absolute path, including filename, of that file. The default value, , will store the file in your project directory.
If you are not using SQLite as your database, additional settings such as , , must be added. For more details, see the reference documentation for .
Note
If you’re using PostgreSQL or MySQL, make sure you’ve created a database by this point. Do that with “” within your database’s interactive prompt.
If you’re using SQLite, you don’t need to create anything beforehand - the database file will be created automatically when it is needed.
While you’re editing , set to your time zone.
Also, note the setting at the top of the file. That holds the names of all Django applications that are activated in this Django instance. Apps can be used in multiple projects, and you can package and distribute them for use by others in their projects.
By default, contains the following apps, all of which come with Django:
These applications are included by default as a convenience for the common case.
Some of these applications make use of at least one database table, though, so we need to create the tables in the database before we can use them. To do that, run the following command:
The command looks at the setting and creates any necessary database tables according to the database settings in your file and the database migrations shipped with the app (we’ll cover those later). You’ll see a message for each migration it applies. If you’re interested, run the command-line client for your database and type (PostgreSQL), (MySQL), (SQLite), or (Oracle) to display the tables Django created.
For the minimalists
Like we said above, the default applications are included for the common case, but not everybody needs them. If you don’t need any or all of them, feel free to comment-out or delete the appropriate line(s) from before running . The command will only run migrations for apps in .
The development server¶
Let’s verify your Django project works. Change into the outer directory, if you haven’t already, and run the following commands:
You’ll see the following output on the command line:
Performing system checks... 0 errors found January 08, 2020 - 15:50:53 Django version 1.8, using settings 'mysite.settings' Starting development server at http://127.0.0.1:8000/ Quit the server with CONTROL-C.You’ve started the Django development server, a lightweight Web server written purely in Python. We’ve included this with Django so you can develop things rapidly, without having to deal with configuring a production server – such as Apache – until you’re ready for production.
Now’s a good time to note: don’t use this server in anything resembling a production environment. It’s intended only for use while developing. (We’re in the business of making Web frameworks, not Web servers.)
Now that the server’s running, visit http://127.0.0.1:8000/ with your Web browser. You’ll see a “Welcome to Django” page, in pleasant, light-blue pastel. It worked!
Changing the port
By default, the command starts the development server on the internal IP at port 8000.
If you want to change the server’s port, pass it as a command-line argument. For instance, this command starts the server on port 8080:
If you want to change the server’s IP, pass it along with the port. So to listen on all public IPs (useful if you want to show off your work on other computers on your network), use:
Full docs for the development server can be found in the reference.
Automatic reloading of
The development server automatically reloads Python code for each request as needed. You don’t need to restart the server for code changes to take effect. However, some actions like adding files don’t trigger a restart, so you’ll have to restart the server in these cases.
Creating models¶
Now that your environment – a “project” – is set up, you’re set to start doing work.
Each application you write in Django consists of a Python package that follows a certain convention. Django comes with a utility that automatically generates the basic directory structure of an app, so you can focus on writing code rather than creating directories.
Projects vs. apps
What’s the difference between a project and an app? An app is a Web application that does something – e.g., a Weblog system, a database of public records or a simple poll app. A project is a collection of configuration and apps for a particular Web site. A project can contain multiple apps. An app can be in multiple projects.
Your apps can live anywhere on your Python path. In this tutorial, we’ll create our poll app right next to your file so that it can be imported as its own top-level module, rather than a submodule of .
To create your app, make sure you’re in the same directory as and type this command:
That’ll create a directory , which is laid out like this:
This directory structure will house the poll application.
The first step in writing a database Web app in Django is to define your models – essentially, your database layout, with additional metadata.
Philosophy
A model is the single, definitive source of truth about your data. It contains the essential fields and behaviors of the data you’re storing. Django follows the DRY Principle. The goal is to define your data model in one place and automatically derive things from it.
This includes the migrations - unlike in Ruby On Rails, for example, migrations are entirely derived from your models file, and are essentially just a history that Django can roll through to update your database schema to match your current models.
In our simple poll app, we’ll create two models: and . A has a question and a publication date. A has two fields: the text of the choice and a vote tally. Each is associated with a .
These concepts are represented by simple Python classes. Edit the file so it looks like this:
The code is straightforward. Each model is represented by a class that subclasses . Each model has a number of class variables, each of which represents a database field in the model.
Each field is represented by an instance of a class – e.g., for character fields and for datetimes. This tells Django what type of data each field holds.
The name of each instance (e.g. or ) is the field’s name, in machine-friendly format. You’ll use this value in your Python code, and your database will use it as the column name.
You can use an optional first positional argument to a to designate a human-readable name. That’s used in a couple of introspective parts of Django, and it doubles as documentation. If this field isn’t provided, Django will use the machine-readable name. In this example, we’ve only defined a human-readable name for . For all other fields in this model, the field’s machine-readable name will suffice as its human-readable name.
Some classes have required arguments. , for example, requires that you give it a . That’s used not only in the database schema, but in validation, as we’ll soon see.
A can also have various optional arguments; in this case, we’ve set the value of to 0.
Finally, note a relationship is defined, using . That tells Django each is related to a single . Django supports all the common database relationships: many-to-one, many-to-many and one-to-one.
Activating models¶
That small bit of model code gives Django a lot of information. With it, Django is able to:
- Create a database schema ( statements) for this app.
- Create a Python database-access API for accessing and objects.
But first we need to tell our project that the app is installed.
Philosophy
Django apps are “pluggable”: You can use an app in multiple projects, and you can distribute apps, because they don’t have to be tied to a given Django installation.
Edit the file again, and change the setting to include the string . So it’ll look like this:
Now Django knows to include the app. Let’s run another command:
You should see something similar to the following:
By running , you’re telling Django that you’ve made some changes to your models (in this case, you’ve made new ones) and that you’d like the changes to be stored as a migration.
Migrations are how Django stores changes to your models (and thus your database schema) - they’re just files on disk. You can read the migration for your new model if you like; it’s the file . Don’t worry, you’re not expected to read them every time Django makes one, but they’re designed to be human-editable in case you want to manually tweak how Django changes things.
There’s a command that will run the migrations for you and manage your database schema automatically - that’s called , and we’ll come to it in a moment - but first, let’s see what SQL that migration would run. The command takes migration names and returns their SQL:
You should see something similar to the following (we’ve reformatted it for readability):
Note the following:
- The exact output will vary depending on the database you are using. The example above is generated for PostgreSQL.
- Table names are automatically generated by combining the name of the app () and the lowercase name of the model – and . (You can override this behavior.)
- Primary keys (IDs) are added automatically. (You can override this, too.)
- By convention, Django appends to the foreign key field name. (Yes, you can override this, as well.)
- The foreign key relationship is made explicit by a constraint. Don’t worry about the parts; that’s just telling PostgreSQL to not enforce the foreign key until the end of the transaction.
- It’s tailored to the database you’re using, so database-specific field types such as (MySQL), (PostgreSQL), or (SQLite) are handled for you automatically. Same goes for the quoting of field names – e.g., using double quotes or single quotes.
- The command doesn’t actually run the migration on your database - it just prints it to the screen so that you can see what SQL Django thinks is required. It’s useful for checking what Django is going to do or if you have database administrators who require SQL scripts for changes.
If you’re interested, you can also run ; this checks for any problems in your project without making migrations or touching the database.
Now, run again to create those model tables in your database:
The command takes all the migrations that haven’t been applied (Django tracks which ones are applied using a special table in your database called ) and runs them against your database - essentially, synchronizing the changes you made to your models with the schema in the database.
Migrations are very powerful and let you change your models over time, as you develop your project, without the need to delete your database or tables and make new ones - it specializes in upgrading your database live, without losing data. We’ll cover them in more depth in a later part of the tutorial, but for now, remember the three-step guide to making model changes:
The reason that there are separate commands to make and apply migrations is because you’ll commit migrations to your version control system and ship them with your app; they not only make your development easier, they’re also useable by other developers and in production.
Read the django-admin documentation for full information on what the utility can do.
Playing with the API¶
Now, let’s hop into the interactive Python shell and play around with the free API Django gives you. To invoke the Python shell, use this command:
We’re using this instead of simply typing “python”, because sets the environment variable, which gives Django the Python import path to your file.
Bypassing manage.py
If you’d rather not use , no problem. Just set the environment variable to , start a plain Python shell, and set up Django:
If this raises an , you’re probably using a version of Django that doesn’t match this tutorial version. You’ll want to either switch to the older tutorial or the newer Django version.
You must run from the same directory is in, or ensure that directory is on the Python path, so that works.
For more information on all of this, see the django-admin documentation.
Once you’re in the shell, explore the database API:
Wait a minute. is, utterly, an unhelpful representation of this object. Let’s fix that by editing the model (in the file) and adding a method to both and :
It’s important to add methods to your models, not only for your own convenience when dealing with the interactive prompt, but also because objects’ representations are used throughout Django’s automatically-generated admin.
or ?
On Python 3, it’s easy, just use .
On Python 2, you should define methods returning values instead. Django models have a default method that calls and converts the result to a UTF-8 bytestring. This means that will return a Unicode string, and will return a bytestring, with characters encoded as UTF-8. Python does the opposite: has a method that calls and interprets the result as an ASCII bytestring. This difference can create confusion.
If all of this is gibberish to you, just use Python 3.
Note these are normal Python methods. Let’s add a custom method, just for demonstration:
Note the addition of and , to reference Python’s standard module and Django’s time-zone-related utilities in , respectively. If you aren’t familiar with time zone handling in Python, you can learn more in the time zone support docs.
Save these changes and start a new Python interactive shell by running again:
For more information on model relations, see Accessing related objects. For more on how to use double underscores to perform field lookups via the API, see Field lookups. For full details on the database API, see our Database API reference.
When you’re comfortable with the API, read part 2 of this tutorial to get Django’s automatic admin working.
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