It should be simple, bit I've spent the last hour searching for the answer. This is using psycopg2 on python 2.6.
I need something like this:
special_id = 5
sql = """
select count(*) as ct,
from some_table tbl
where tbl.id = %(the_id)
"""
cursor = connection.cursor()
cursor.execute(sql, {"the_id" : special_id})
I cannot get this to work. Were special_id a string, I could replace %(the_id) with %(the_id)s and things work well. However, I want it to use the integer so that it hits my indexes correctly.
There is a surprising lack of specific information on psycopg2 on the internet. I hope someone has an answer to this seemingly simple question.
Per PEP 249, since in psycopg2 paramstyle is pyformat, you need to use %(the_id)s even for non-strings -- trust it to do the right thing.
BTW, internet searches will work better if you use the correct spelling (no h there), but even if you mis-spelled, I'm surprised you didn't get a "did you mean" hint (I did when I deliberately tried!).
Related
I'm to link my code to a MySQL database using pymysql. In general everything has gone smoothly but I'm having difficulty with the following function to find the minimum of a variable column.
def findmin(column):
cur = db.cursor()
sql = "SELECT MIN(%s) FROM table"
cur.execute(sql,column)
mintup = cur.fetchone()
if everything went smoothly this would return me a tuple with the minimum, e.g. (1,).
However, if I run the function:
findmin(column_name)
I have to put column name in "" (i.e. "column_name"), else Python sees it as an unknown variable. But if I put the quotation marks around column_name then SQL sees
SELECT MIN("column_name") FROM table
which just returns the column header, not the value.
How can I get around this?
The issue is likely the use of %s for the column name. That means the SQL Driver will try to escape that variable when interpolating it, including quoting, which is not what you want for things like column names, table names, etc.
When using a value in SELECT, WHERE, etc. then you do want to use %s to prevent SQL injections and enable quoting, among other things.
Here, you just want to interpolate using pure Python (assuming a trusted value; please see below for more information). That also means no bindings tuple passed to the execute method.
def findmin(column):
cur = db.cursor()
sql = "SELECT MIN({0}) FROM table".format(column)
cur.execute(sql)
mintup = cur.fetchone()
SQL fiddle showing the SQL working:
http://sqlfiddle.com/#!2/e70a41/1
In response to the Jul 15, 2014 comment from Colin Phipps (September 2022):
The relatively recent edit on this post by another community member brought it to my attention, and I wanted to respond to Colin's comment from many years ago.
I totally agree re: being careful about one's input if one interpolates like this. Certainly one needs to know exactly what is being interpolated. In this case, I would say a defined value within a trusted internal script or one supplied by a trusted internal source would be fine. But if, as Colin mentioned, there is any external input, then that is much different and additional precautions should be taken.
Pretty new to sqlite3, so bear with me here..
I'd like to have a function to which I can pass the table name, and the values to update.
I initially started with something like this:
def add_to_table(table_name, string):
cursor.execute('INSERT INTO {table} VALUES ({var})'
.format(
table=table_name,
var=string)
)
Which works A-OK, but further reading about sqlite3 suggested that this was a terribly insecure way to go about things. However, using their ? syntax, I'm unable to pass in a name to specify the variable.
I tried adding in a ? in place of the table, but that throws a syntax error.
cursor.execute('INSERT INTO ? VALUES (?)', ('mytable','"Jello, world!"'))
>> >sqlite3.OperationalError: near "?": syntax error
Can the table in an sql statement be passed in safely and dynamically?
Its not the dynamic string substitution per-se thats the problem. Its dynamic string substitution with an user-supplied string thats the big problem because that opens you to SQL-injection attacks. If you are absolutely 100% sure that the tablename is a safe string that you control then splicing it into the SQL query will be safe.
if some_condition():
table_name = 'TABLE_A'
else:
table_name = 'TABLE_B'
cursor.execute('INSERT INTO '+ table_name + 'VALUES (?)', values)
That said, using dynamic SQL like that is certainly a code smell so you should double check to see if you can find a simpler alternative without the dynamically generated SQL strings. Additionally, if you really want dynamic SQL then something like SQLAlchemy might be useful to guarantee that the SQL you generate is well formed.
Composing SQL statements using string manipulation is odd not only because of security implications, but also because strings are "dumb" objects. Using sqlalchemy core (you don't even need the ORM part) is almost like using strings, but each fragment will be a lot smarter and allow for easier composition. Take a look at the sqlalchemy wiki to get a notion of what I'm talking about.
For example, using sqlsoup your code would look like this:
db = SQLSoup('sqlite://yourdatabase')
table = getattr(db, tablename)
table.insert(fieldname='value', otherfield=123)
db.commit()
Another advantage: code is database independent - want to move to oracle? Change the connection string and you are done.
I'm to link my code to a MySQL database using pymysql. In general everything has gone smoothly but I'm having difficulty with the following function to find the minimum of a variable column.
def findmin(column):
cur = db.cursor()
sql = "SELECT MIN(%s) FROM table"
cur.execute(sql,column)
mintup = cur.fetchone()
if everything went smoothly this would return me a tuple with the minimum, e.g. (1,).
However, if I run the function:
findmin(column_name)
I have to put column name in "" (i.e. "column_name"), else Python sees it as an unknown variable. But if I put the quotation marks around column_name then SQL sees
SELECT MIN("column_name") FROM table
which just returns the column header, not the value.
How can I get around this?
The issue is likely the use of %s for the column name. That means the SQL Driver will try to escape that variable when interpolating it, including quoting, which is not what you want for things like column names, table names, etc.
When using a value in SELECT, WHERE, etc. then you do want to use %s to prevent SQL injections and enable quoting, among other things.
Here, you just want to interpolate using pure Python (assuming a trusted value; please see below for more information). That also means no bindings tuple passed to the execute method.
def findmin(column):
cur = db.cursor()
sql = "SELECT MIN({0}) FROM table".format(column)
cur.execute(sql)
mintup = cur.fetchone()
SQL fiddle showing the SQL working:
http://sqlfiddle.com/#!2/e70a41/1
In response to the Jul 15, 2014 comment from Colin Phipps (September 2022):
The relatively recent edit on this post by another community member brought it to my attention, and I wanted to respond to Colin's comment from many years ago.
I totally agree re: being careful about one's input if one interpolates like this. Certainly one needs to know exactly what is being interpolated. In this case, I would say a defined value within a trusted internal script or one supplied by a trusted internal source would be fine. But if, as Colin mentioned, there is any external input, then that is much different and additional precautions should be taken.
Pretty new to sqlite3, so bear with me here..
I'd like to have a function to which I can pass the table name, and the values to update.
I initially started with something like this:
def add_to_table(table_name, string):
cursor.execute('INSERT INTO {table} VALUES ({var})'
.format(
table=table_name,
var=string)
)
Which works A-OK, but further reading about sqlite3 suggested that this was a terribly insecure way to go about things. However, using their ? syntax, I'm unable to pass in a name to specify the variable.
I tried adding in a ? in place of the table, but that throws a syntax error.
cursor.execute('INSERT INTO ? VALUES (?)', ('mytable','"Jello, world!"'))
>> >sqlite3.OperationalError: near "?": syntax error
Can the table in an sql statement be passed in safely and dynamically?
Its not the dynamic string substitution per-se thats the problem. Its dynamic string substitution with an user-supplied string thats the big problem because that opens you to SQL-injection attacks. If you are absolutely 100% sure that the tablename is a safe string that you control then splicing it into the SQL query will be safe.
if some_condition():
table_name = 'TABLE_A'
else:
table_name = 'TABLE_B'
cursor.execute('INSERT INTO '+ table_name + 'VALUES (?)', values)
That said, using dynamic SQL like that is certainly a code smell so you should double check to see if you can find a simpler alternative without the dynamically generated SQL strings. Additionally, if you really want dynamic SQL then something like SQLAlchemy might be useful to guarantee that the SQL you generate is well formed.
Composing SQL statements using string manipulation is odd not only because of security implications, but also because strings are "dumb" objects. Using sqlalchemy core (you don't even need the ORM part) is almost like using strings, but each fragment will be a lot smarter and allow for easier composition. Take a look at the sqlalchemy wiki to get a notion of what I'm talking about.
For example, using sqlsoup your code would look like this:
db = SQLSoup('sqlite://yourdatabase')
table = getattr(db, tablename)
table.insert(fieldname='value', otherfield=123)
db.commit()
Another advantage: code is database independent - want to move to oracle? Change the connection string and you are done.
For some reasons, I would like to do an explicit quoting of a string value (becoming a part of constructed SQL query) instead of waiting for implicit quotation performed by cursor.execute method on contents of its second parameter.
By "implicit quotation" I mean:
value = "Unsafe string"
query = "SELECT * FROM some_table WHERE some_char_field = %s;"
cursor.execute( query, (value,) ) # value will be correctly quoted
I would prefer something like that:
value = "Unsafe string"
query = "SELECT * FROM some_table WHERE some_char_field = %s;" % \
READY_TO_USE_QUOTING_FUNCTION(value)
cursor.execute( query ) # value will be correctly quoted, too
Is such low level READY_TO_USE_QUOTING_FUNCTION expected by Python DB API specification (I couldn't find such functionality in PEP 249 document). If not, maybe Psycopg2 provides such function? If not, maybe Django provides such function? I would prefer not to write such function myself...
Ok, so I was curious and went and looked at the source of psycopg2. Turns out I didn't have to go further than the examples folder :)
And yes, this is psycopg2-specific. Basically, if you just want to quote a string you'd do this:
from psycopg2.extensions import adapt
print adapt("Hello World'; DROP DATABASE World;")
But what you probably want to do is to write and register your own adapter;
In the examples folder of psycopg2 you find the file 'myfirstrecipe.py' there is an example of how to cast and quote a specific type in a special way.
If you have objects for the stuff you want to do, you can just create an adapter that conforms to the 'IPsycopgSQLQuote' protocol (see pydocs for the myfirstrecipe.py-example...actually that's the only reference I can find to that name) that quotes your object and then registering it like so:
from psycopg2.extensions import register_adapter
register_adapter(mytype, myadapter)
Also, the other examples are interesting; esp. 'dialtone.py' and 'simple.py'.
I guess you're looking for the mogrify function.
Example:
>>> cur.mogrify("INSERT INTO test (num, data) VALUES (%s, %s)", (42, 'bar'))
"INSERT INTO test (num, data) VALUES (42, E'bar')"
You should try to avoid doing your own quoting. Not only will it be DB-specific as people have pointed out, but flaws in quoting are the source of SQL injection bugs.
If you don't want to pass around queries and values separately, then pass around a list of the parameters:
def make_my_query():
# ...
return sql, (value1, value2)
def do_it():
query = make_my_query()
cursor.execute(*query)
(I probably have the syntax of cursor.execute wrong) The point here is that just because cursor.execute takes a number of arguments, that doesn't mean you have to handle them all separately. You can deal with them as one list.
This'll be database dependent (iirc, mysql allows \ as an escape character, while something like oracle expects quotes to be doubled: 'my '' quoted string').
Someone correct me if i'm wrong, but the double-quoting method is the standard method.
It may be worth looking at what other db abstraction libraries do (sqlalchemy, cx_Oracle, sqlite, etc).
I've got to ask - why do you want to inline the values instead of bind them?
This is going to be DB dependent. In the case of MySQLdb, for example, the connection class has a literal method that will convert the value to the correct escaped representation for passing to MySQL (that's what cursor.execute uses).
I imagine Postgres has something similar, but I don't think there is a function to escape values as part of the DB API 2.0 spec.
I don't think you give any sufficient reasoning behind your avoidance to do this The Right Way. Please, use the APi as it is designed and don't try so hard to make your code less readable for the next guy and more fragile.
Your code snippet would get just like this, according to psycopg extension docs
from psycopg2.extensions import adapt
value = "Unsafe string"
query = "SELECT * FROM some_table WHERE some_char_field = %s;" % \
adapt(value).getquoted()
cursor.execute( query ) # value will be correctly quoted, too
The getquoted function returns the value as a quoted and escaped string, so you could also go: "SELECT * FROM some_table WHERE some_char_field = " + adapt(value).getquoted() .
PyPika in another good option for building SQL statements. Usage example (based on an example on the project's homepage):
>>> from pypika import Order, Query
>>> Query.from_('customers').select('id', 'fname', 'lname', 'phone').orderby('id', order=Order.desc)
SELECT "id","fname","lname","phone" FROM "customers" ORDER BY "id" DESC
If you use django you might want to use the quoting function which is automatically adapted to the currently configured DBMS :
from django.db import backend
my_quoted_variable = backend.DatabaseOperations().quote_name(myvar)
import re
def db_quote(s):
return "\"" + re.escape(s) + "\""
can do the job of simple quoting that works at least with MySQL. What we really need, though is cursor.format() function that would work like cursor.execute() except it would return the resulting query instead of executing it. There are times when you do not want the query to be executed quite yet - e.g you may want to log it first, or print it out for debugging before you go ahead with it.