Introduction

MySQLdb is an thread-compatible interface to the popular MySQL
database server that provides the Python database API.

Installation

The README
file has complete installation instructions.

_mysql

If you want to write applications which are portable across
databases,
use MySQLdb,
and avoid using this module directly. _mysql
provides an interface which mostly implements the MySQL C API. For
more information, see the MySQL documentation. The
documentation
for this module is intentionally weak because you probably should use
the higher-level MySQLdb module. If you really need it, use the
standard MySQL docs and transliterate as necessary.

MySQL C API
translation

The MySQL C API has been wrapped in an object-oriented way. The only
MySQL data structures which are implemented are the MYSQL
(database connection handle) and MYSQL_RES (result handle)
types. In general, any function which takes MYSQL *mysql as an
argument is now a method of the connection object, and any function
which takes MYSQL_RES
*result
as an argument is a method of the
result object. Functions requiring none of the MySQL data structures
are implemented as functions in the module. Functions requiring one of
the other MySQL data structures are generally not implemented.
Deprecated functions are not implemented. In all cases, the mysql_
prefix is dropped from the name. Most of the conn methods listed
are also available as MySQLdb Connection object methods. Their use is
non-portable.

MySQL
C API function mapping

C API _mysql
mysql_affected_rows() conn.affected_rows()
mysql_autocommit() conn.autocommit()
mysql_character_set_name() conn.character_set_name()
mysql_close() conn.close()
mysql_commit() conn.commit()
mysql_connect() _mysql.connect()
mysql_data_seek() result.data_seek()
mysql_debug() _mysql.debug()
mysql_dump_debug_info conn.dump_debug_info()
mysql_escape_string() _mysql.escape_string()
mysql_fetch_row() result.fetch_row()
mysql_get_character_set_info() conn.get_character_set_info()
mysql_get_client_info() _mysql.get_client_info()
mysql_get_host_info() conn.get_host_info()
mysql_get_proto_info() conn.get_proto_info()
mysql_get_server_info() conn.get_server_info()
mysql_info() conn.info()
mysql_insert_id() conn.insert_id()
mysql_num_fields() result.num_fields()
mysql_num_rows() result.num_rows()
mysql_options() various options to _mysql.connect()
mysql_ping() conn.ping()
mysql_query() conn.query()
mysql_real_connect() _mysql.connect()
mysql_real_query() conn.query()
mysql_real_escape_string() conn.escape_string()
mysql_rollback() conn.rollback()
mysql_row_seek() result.row_seek()
mysql_row_tell() result.row_tell()
mysql_select_db() conn.select_db()
mysql_set_character_set() conn.set_character_set()
mysql_ssl_set() ssl
option to _mysql.connect()
mysql_stat() conn.stat()
mysql_store_result() conn.store_result()
mysql_thread_id() conn.thread_id()
mysql_thread_safe_client() conn.thread_safe_client()
mysql_use_result() conn.use_result()
mysql_warning_count() conn.warning_count()
CLIENT_* MySQLdb.constants.CLIENT.*
CR_* MySQLdb.constants.CR.*
ER_* MySQLdb.constants.ER.*
FIELD_TYPE_* MySQLdb.constants.FIELD_TYPE.*
FLAG_* MySQLdb.constants.FLAG.*

Some _mysql examples

Okay, so you want to use _mysql anyway. Here are some examples.

The simplest possible database connection is:

This creates a connection to the MySQL server running on the local
machine using the standard UNIX socket (or named pipe on Windows),
your login name (from the USER environment variable), no password, and
does not USE a
database. Chances are you need to supply more
information.:

This creates a connection to the MySQL server running on the local
machine via a UNIX socket (or named pipe), the user name “joebob”, the
password “moonpie”, and selects the initial database “thangs”.

We haven’t even begun to touch upon all the parameters connect()
can take. For this reason, I prefer to use keyword parameters:

This does exactly what the last example did, but is arguably easier
to
read. But since the default host is “localhost”, and if your login
name really was “joebob”, you could shorten it to this:

UNIX sockets and named pipes don’t work over a network, so if you
specify a host other than localhost, TCP will be used, and you can
specify an odd port if you need to (the default port is 3306):

If you really had to, you could connect to the local host with TCP by
specifying the full host name, or 127.0.0.1.

Generally speaking, putting passwords in your code is not such a good
idea:

This does what the previous example does, but gets the username and
password and other parameters from ~/.my.cnf (UNIX-like systems). Read
about option files
for more details.

So now you have an open connection as db and want to do a
query. Well, there are no cursors in MySQL, and no parameter
substitution, so you have to pass a complete query string to
db.query():

There’s no return value from this, but exceptions can be raised. The
exceptions are defined in a separate module, _mysql_exceptions,
but _mysql
exports them. Read DB API specification PEP-249 to
find out what they are, or you can use the catch-all MySQLError.

At this point your query has been executed and you need to get the
results. You have two options:

Both methods return a result object. What’s the difference?
store_result()
returns the entire result set to the client
immediately. If your result set is really large, this could be a
problem. One way around this is to add a LIMIT clause to your
query, to limit the number of rows returned. The other is to use
use_result(),
which keeps the result set in the server and sends
it row-by-row when you fetch. This does, however, tie up server
resources, and it ties up the connection: You cannot do any more
queries until you have fetched all the rows. Generally I
recommend using store_result()
unless your result set is really
huge and you can’t use LIMIT
for some reason.

Now, for actually getting real results:

This might look a little odd. The first thing you should know is,
fetch_row()
takes some additional parameters. The first one is,
how many rows (maxrows)
should be returned. By default, it returns
one row. It may return fewer rows than you asked for, but never
more. If you set maxrows=0,
it returns all rows of the result
set. If you ever get an empty tuple back, you ran out of rows.

The second parameter (how)
tells it how the row should be
represented. By default, it is zero which means, return as a tuple.
how=1 means,
return it as a dictionary, where the keys are the
column names, or table.column
if there are two columns with the
same name (say, from a join). how=2 means the same as how=1
except that the keys are always table.column; this is for
compatibility with the old Mysqldb module.

OK, so why did we get a 1-tuple with a tuple inside? Because we
implicitly asked for one row, since we didn’t specify maxrows.

The other oddity is: Assuming these are numeric columns, why are they
returned as strings? Because MySQL returns all data as strings and
expects you to convert it yourself. This would be a real pain in the
ass, but in fact, _mysql
can do this for you. (And MySQLdb
does do this for you.) To have automatic type conversion done, you
need to create a type converter dictionary, and pass this to
connect() as
the conv
keyword parameter.

The keys of conv
should be MySQL column types, which in the
C API are FIELD_TYPE_*.
You can get these values like this:

By default, any column type that can’t be found in conv is
returned as a string, which works for a lot of stuff. For our
purposes, we probably want this:

This means, if it’s a FIELD_TYPE_LONG,
call the builtin int()
function on it. Note that FIELD_TYPE_LONG is an INTEGER
column, which corresponds to a C long, which is also the type used
for a normal Python integer. But beware: If it’s really an UNSIGNED
INTEGER
column, this could cause
overflows. For this reason,
MySQLdb
actually uses long()
to do the conversion. But we’ll
ignore this potential problem for now.

Then if you use db=_mysql.connect(conv=my_conv...),
the
results will come back ((3,
2, 0),)
, which
is what you would
expect.

MySQLdb

MySQLdb is a thin Python wrapper around _mysql which makes it
compatible with the Python DB API interface (version 2). In reality,
a fair amount of the code which implements the API is in _mysql
for the sake of efficiency.

The DB API specification PEP-249 should be
your primary guide for
using this module. Only deviations from the spec and other
database-dependent things will be documented here.

Functions
and attributes

Only a few top-level functions and attributes are defined within
MySQLdb.

connect(parameters…)

Constructor for creating a connection to the
database. Returns a Connection Object. Parameters are the
same as for the MySQL C API. In addition, there are a few
additional keywords that correspond to what you would pass
mysql_options()
before connecting. Note that some
parameters must be specified as keyword arguments! The
default value for each parameter is NULL or zero, as
appropriate. Consult the MySQL documentation for more
details. The important parameters are:

host
name of host to connect to. Default: use the local host
via a UNIX socket (where applicable)
user
user to authenticate as. Default: current effective user.
passwd
password to authenticate with. Default: no password.
db
database to use. Default: no default database.
port
TCP port of MySQL server. Default: standard port (3306).
unix_socket
location of UNIX socket. Default: use default location or
TCP for remote hosts.
conv
type conversion dictionary. Default: a copy of
MySQLdb.converters.conversions
compress
Enable protocol compression. Default: no compression.
connect_timeout
Abort if connect is not completed within
given number of seconds. Default: no timeout (?)
named_pipe
Use a named pipe (Windows). Default: don’t.
init_command
Initial command to issue to server upon
connection. Default: Nothing.
read_default_file
MySQL configuration file to read; see
the MySQL documentation for mysql_options().
read_default_group
Default group to read; see the MySQL
documentation for mysql_options().
cursorclass
cursor class that cursor()
uses, unless
overridden. Default: MySQLdb.cursors.Cursor.
This
must be a keyword parameter.
use_unicode

If True, CHAR and VARCHAR and TEXT columns are
returned as
Unicode strings, using the configured character set. It is
best to set the default encoding in the server
configuration, or client configuration (read with
read_default_file). If you change the character set after
connecting (MySQL-4.1 and later), you’ll need to put the
correct character set name in connection.charset.

If False, text-like columns are returned as normal strings,
but you can always write Unicode strings.

This must be a keyword parameter.

charset

If present, the connection character set will be
changed
to this character set, if they are not equal. Support for
changing the character set requires MySQL-4.1 and later
server; if the server is too old, UnsupportedError will be
raised. This option implies use_unicode=True, but you can
override this with use_unicode=False, though you probably
shouldn’t.

If not present, the default character set is used.

This must be a keyword parameter.

sql_mode

If present, the session SQL mode will be set to the
given
string. For more information on sql_mode, see the MySQL
documentation. Only available for 4.1 and newer servers.

If not present, the session SQL mode will be unchanged.

This must be a keyword parameter.

ssl
This parameter takes a dictionary or mapping, where the
keys are parameter names used by the mysql_ssl_set
MySQL
C API call. If this is set, it initiates an SSL connection
to the server; if there is no SSL support in the client,
an exception is raised. This must be a keyword
parameter.
apilevel
String constant stating the supported DB API level. ‘2.0’
threadsafety

Integer constant stating the level of thread safety
the
interface supports. This is set to 1, which means: Threads may
share the module.

The MySQL protocol can not handle multiple threads using the
same connection at once. Some earlier versions of MySQLdb
utilized locking to achieve a threadsafety of 2. While this is
not terribly hard to accomplish using the standard Cursor class
(which uses mysql_store_result()),
it is complicated by
SSCursor (which uses mysql_use_result();
with the latter you
must ensure all the rows have been read before another query can
be executed. It is further complicated by the addition of
transactions, since transactions start when a cursor execute a
query, but end when COMMIT
or ROLLBACK
is executed by
the Connection object. Two threads simply cannot share a
connection while a transaction is in progress, in addition to
not being able to share it during query execution. This
excessively complicated the code to the point where it just
isn’t worth it.

The general upshot of this is: Don’t share connections between
threads. It’s really not worth your effort or mine, and in the
end, will probably hurt performance, since the MySQL server runs
a separate thread for each connection. You can certainly do
things like cache connections in a pool, and give those
connections to one thread at a time. If you let two threads use
a connection simultaneously, the MySQL client library will
probably upchuck and die. You have been warned.

For threaded applications, try using a connection pool.
This can be done using the Pool module.

charset
The character set used by the connection. In MySQL-4.1 and newer,
it is possible (but not recommended) to change the connection’s
character set with an SQL statement. If you do this, you’ll also
need to change this attribute. Otherwise, you’ll get encoding
errors.
paramstyle

String constant stating the type of parameter
marker formatting
expected by the interface. Set to ‘format’ = ANSI C printf
format codes, e.g. ‘…WHERE name=%s’. If a mapping object is
used for conn.execute(), then the interface actually uses
‘pyformat’ = Python extended format codes, e.g. ‘…WHERE
name=%(name)s’. However, the API does not presently allow the
specification of more than one style in paramstyle.

Note that any literal percent signs in the query string passed
to execute() must be escaped, i.e. %%.

Parameter placeholders can only be used
to insert column
values. They can not be used for other parts of SQL,
such as
table names, statements, etc.

conv

A dictionary or mapping which controls how types
are converted
from MySQL to Python and vice versa.

If the key is a MySQL type (from FIELD_TYPE.*), then the value
can be either:

  • a callable object which takes a string argument (the MySQL
    value),’ returning a Python value
  • a sequence of 2-tuples, where the first value is a combination
    of flags from MySQLdb.constants.FLAG,
    and the second value
    is a function as above. The sequence is tested until the flags
    on the field match those of the first value. If both values
    are None, then the default conversion is done. Presently this
    is only used to distinquish TEXT and BLOB columns.

If the key is a Python type or class, then the value is a
callable Python object (usually a function) taking two arguments
(value to convert, and the conversion dictionary) which converts
values of this type to a SQL literal string value.

This is initialized with reasonable defaults for most
types. When creating a Connection object, you can pass your own
type converter dictionary as a keyword parameter. Otherwise, it
uses a copy of MySQLdb.converters.conversions.
Several
non-standard types are returned as strings, which is how MySQL
returns all columns. For more details, see the built-in module
documentation.

Connection Objects

Connection objects are returned by the connect() function.

commit()
If the database and the tables support transactions, this
commits the current transaction; otherwise this method
successfully does nothing.
rollback()
If the database and tables support transactions, this rolls back
(cancels) the current transaction; otherwise a
NotSupportedError
is raised.
cursor([cursorclass])
MySQL does not support cursors; however, cursors are easily
emulated. You can supply an alternative cursor class as an
optional parameter. If this is not present, it defaults to the
value given when creating the connection object, or the standard
Cursor class.
Also see the additional supplied cursor
classes in the usage section.

There are many more methods defined on the connection object which
are MySQL-specific. For more information on them, consult the internal
documentation using pydoc.

Cursor Objects

callproc(procname, args)

Calls stored procedure procname with the sequence
of arguments
in args. Returns the original arguments. Stored procedure
support only works with MySQL-5.0 and newer.

Compatibility note: PEP-249 specifies
that if there are
OUT or INOUT parameters, the modified values are to be
returned. This is not consistently possible with MySQL. Stored
procedure arguments must be passed as server variables, and
can only be returned with a SELECT statement. Since a stored
procedure may return zero or more result sets, it is impossible
for MySQLdb to determine if there are result sets to fetch
before the modified parmeters are accessible.

The parameters are stored in the server as @_*procname*_*n*,
where n is the position of the parameter. I.e., if you
cursor.callproc(‘foo’, (a, b, c)), the parameters will be
accessible by a SELECT statement as @_foo_0, @_foo_1, and
@_foo_2.

Compatibility note: It appears that the
mere act of
executing the CALL statement produces an empty result set, which
appears after any result sets which might be generated by the
stored procedure. Thus, you will always need to use nextset() to
advance result sets.

close()
Closes the cursor. Future operations raise ProgrammingError.
If you are using server-side cursors, it is very important to
close the cursor when you are done with it and before creating a
new one.
info()
Returns some information about the last query. Normally
you don’t need to check this. If there are any MySQL
warnings, it will cause a Warning to be issued through
the Python warning module. By default, Warning causes a
message to appear on the console. However, it is possible
to filter these out or cause Warning to be raised as exception.
See the MySQL docs for mysql_info(),
and the Python warning
module. (Non-standard)
setinputsizes()
Does nothing, successfully.
setoutputsizes()
Does nothing, successfully.
nextset()

Advances the cursor to the next result set,
discarding the remaining
rows in the current result set. If there are no additional result
sets, it returns None; otherwise it returns a true value.

Note that MySQL doesn’t support multiple result sets
until 4.1.

Some examples

The connect()
method works nearly the same as with _mysql:

To perform a query, you first need a cursor, and then you can execute
queries on it:

In this example, max_price=5
Why, then, use %s
in the
string? Because MySQLdb will convert it to a SQL literal value, which
is the string ‘5’. When it’s finished, the query will actually say,
“…WHERE price < 5”.

Why the tuple? Because the DB API requires you to pass in any
parameters as a sequence. Due to the design of the parser, (max_price)
is interpreted as using algebraic grouping and simply as max_price and
not a tuple. Adding a comma, i.e. (max_price,) forces it to make a
tuple.

And now, the results:

Quite unlike the _mysql
example, this returns a single tuple,
which is the row, and the values are properly converted by default…
except… What’s with the L’s?

As mentioned earlier, while MySQL’s INTEGER column translates
perfectly into a Python integer, UNSIGNED INTEGER could overflow, so
these values are converted to Python long integers instead.

If you wanted more rows, you could use c.fetchmany(n) or
c.fetchall().
These do exactly what you think they do. On
c.fetchmany(n),
the n is
optional and defaults to
c.arraysize,
which is normally 1. Both of these methods return a
sequence of rows, or an empty sequence if there are no more rows. If
you use a weird cursor class, the rows themselves might not be tuples.

Note that in contrast to the above, c.fetchone() returns None
when there are no more rows to fetch.

The only other method you are very likely to use is when you have to
do a multi-row insert:

Here we are inserting three rows of five values. Notice that there is
a mix of types (strings, ints, floats) though we still only use
%s. And also
note that we only included format strings for one
row. MySQLdb picks those out and duplicates them for each row.

Using and extending

In general, it is probably wise to not directly interact with the DB
API except for small applicatons. Databases, even SQL databases, vary
widely in capabilities and may have non-standard features. The DB API
does a good job of providing a reasonably portable interface but some
methods are non-portable. Specifically, the parameters accepted by
connect() are
completely implementation-dependent.

If you believe your application may need to run on several different
databases, the author recommends the following approach, based on
personal experience: Write a simplified API for your application which
implements the specific queries and operations your application needs
to perform. Implement this API as a base class which should be have
few database dependencies, and then derive a subclass from this which
implements the necessary dependencies. In this way, porting your
application to a new database should be a relatively simple matter of
creating a new subclass, assuming the new database is reasonably
standard.

Because MySQLdb’s Connection and Cursor objects are written in
Python,
you can easily derive your own subclasses. There are several Cursor
classes in MySQLdb.cursors:

BaseCursor
The base class for Cursor objects. This does not raise Warnings.
CursorStoreResultMixIn
Causes the Cursor to use the mysql_store_result() function to
get the query result. The entire result set is stored on the
client side.
CursorUseResultMixIn
Causes the cursor to use the mysql_use_result() function to
get the query result. The result set is stored on the server side
and is transferred row by row using fetch operations.
CursorTupleRowsMixIn
Causes the cursor to return rows as a tuple of the column values.

CursorDictRowsMixIn

Causes the cursor to return rows as a dictionary, where the keys
are column names and the values are column values. Note that if
the column names are not unique, i.e., you are selecting from two
tables that share column names, some of them will be rewritten as
table.column.
This can be avoided by using the SQL AS
keyword. (This is yet-another reason not to use * in SQL
queries, particularly where JOIN is involved.)

Cursor
The default cursor class. This class is composed of
CursorWarningMixIn,
CursorStoreResultMixIn,
CursorTupleRowsMixIn,
and BaseCursor,
i.e. it raises
Warning, uses
mysql_store_result(),
and returns rows as
tuples.
DictCursor
Like Cursor
except it returns rows as dictionaries.
SSCursor
A “server-side” cursor. Like Cursor but uses
CursorUseResultMixIn.
Use only if you are dealing with
potentially large result sets.
SSDictCursor
Like SSCursor
except it returns rows as dictionaries.

Embedded Server

Instead of connecting to a stand-alone server over the network,
the embedded server support lets you run a full server right in
your Python code or application server.

If you have built MySQLdb with embedded server support, there
are two additional functions you will need to make use of:

server_init(args, groups)

Initialize embedded server. If this client is not
linked against
the embedded server library, this function does nothing.

args
sequence of command-line arguments
groups
sequence of groups to use in defaults files
server_end()
Shut down embedded server. If not using an embedded server, this
does nothing.

See the MySQL documentation for more information on the embedded
server.

Title: MySQLdb: a Python interface for MySQL
Author: Andy Dustman
Version: $Revision: 421 $

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