Python cx_Oracle bind variables

Question

I am a Python newbie, I am having troubles in the use of bind variables. If I execute the code below everything works fine.

bind= {"var" : "ciao"}
sql = "select * from sometable where somefield = :bind"
cur.prepare(sql)
cur.execute(sql,bind)

Instead if I add another bind variable I obtain an error.

bind= {"var" : "ciao"}
sql = "select * from sometable where somefield = :bind and otherfield = :bind"
cur.prepare(sql)
cur.execute(sql,(bind,bind))

cur.execute(sql,(bind,bind))
Oracle.NotSupportedError: Variable_TypeByValue(): unhandled data

I have solved it with

cur.execute(sql,(bind["var"],bind["var"]))

but I can't understand why the previous command was not ok.

Which is the correct way to use bind variables? I am using cx_Oracle.

Lanneret

Answer 1

You are misusing the binding.

There are three different ways of binding variables with cx_Oracle as one can see here :

1) by passing a tuple to a SQL statement with numbered variables :

sql = "select * from sometable where somefield = :1 and otherfield = :2"
cur.execute(sql, (aValue, anotherValue))

2) By passing keyword arguments to a SQL statement with named variables :

sql = "select * from sometable where somefield = :myField and otherfield = :anotherOne"
cur.execute(sql, myField=aValue, anotherOne=anotherValue)

3) By passing a dictionary to a SQL statement with named variables :

sql = "select * from sometable where somefield = :myField and otherfield = :anotherOne"
cur.execute(sql, {"myField":aValue, "anotherOne":anotherValue})

---

Remarks

Why does your code work then ?

Let's try to understand what happens here :

bind= {"var" : "ciao"}
sql = "select * from sometable where somefield = :bind and otherfield = :bind"
cur.execute(sql,(bind["var"], bind["var"]))

Oracle will understand that it expects one variable. This is a named variable, linked by name bind. You should then give a parameter as a named parameter like this :

cur.execute(sql, bind="ciao")

Or using a dictionary, like that :

cur.execute(sql, {bind:"ciao"})

However, as cx_Oracle receives a tuple instead, it fallbacks in a binding by number, as if your SQL statement was :

sql = "select * from sometable where somefield = :1 and otherfield = :2"

And as you are passing bind['var'] twice, which is just the string "ciao". It is mapping the two tuple items to the numbered variables :

cur.execute(sql, ("ciao", "ciao"))

That runs by chance but the code is very misleading.

Tuple with a single value to bind

Also note that the first option requires a tuple. But if you have a single value to bind, you can use this notation to create a tuple of a single value :

sql = "select * from sometable where somefield = :1"
cur.execute(sql, (aValue,))

[EDIT] : Thanks to @tyler-christian for mentioning that passing a dict was supported by cx_Oracle.

Anomalistic

For me it was missing the trailing comma in a single-item tuple. D'oh! – Cyme

@Rob, indeed : type((1,)) = <type 'tuple'>, but type((1)) = <type 'int'> – Anomalistic

In the second and third example, is there a typo in the SQL statement (should my_field be myField) or does this not matter at all? – Wheal

Answer 2

@ffarquest says that using a dictionary isn't supported by cx_Oracle but it is in fact, @giovanni-de-ciantis was just using it incorrectly.

---

named_params = {'dept_id':50, 'sal':1000}
query1 = cursor.execute(
    'SELECT * FROM employees WHERE department_id=:dept_id AND salary>:sal',
    named_params
)

OR

query2 = cursor.execute(
    'SELECT * FROM employees WHERE department_id=:dept_id AND salary>:sal',
    dept_id=50,
    sal=1000
)

---

In the given example, I believe the second reference to :bind would need to be replaced with something different because it isn't being done in sequential order. Also, renamed the variable bind to get rid of confusion.

bind_dict = {bind:"var" : diff:"ciao"}
sql = "select * from sometable where somefield=:bind and otherfield=:diff"
cur.prepare(sql)
cur.execute(sql, bind_dict )

---

This article is from 2007 showing that you can use a dictionary: http://www.oracle.com/technetwork/articles/dsl/prez-python-queries-101587.html

Ailurophile

Thanks for pointing to the fact that using a dictionary is actually possible. My answer has been updated accordingly. – Anomalistic