How to Fix the OperationalError in Python

OperationalError is a class of error encountered while working with the psycopg library in Python. Operational errors are closely linked with the connect method within psycopg and typically occur when parameters passed to this method are incorrect or invalid. This may mean that a parameter, such as a database name, is spelled incorrectly, has changed, or that the database server itself is experiencing a problem.

Example 1: Database Connection Request

Let's try connecting to a database to identify the sources of these errors. For this example, assume the database and user are named postgres, the host is localhost and the password is pa55w0rd.

conn = psycopg2.connect("dbname='postgres' user='postgres' host='foreignhost' password='pa55w0rd'")
cur = conn.cursor()

An Output of Example 1

Notice here that the hostname is entered as foreignhost rather than the correct value of localhost. This results in the method being unable to connect to the database server and producing the following console output.

C:\code\Python\PostreSQL\venv\3K\Scripts\py
C:/code/python/PostreSQL/unique_violation
Traceback (most recent call last):
  File "C:/code/python/PostreSQL/unique_violation", line 12, in <module>
    conn = psycopg2.connect("dbname='postgres' user='postgres' host='foreignhost' password='pa55w0rd'")
  File "C:\code\Python\PostreSQL\venv\3K\lib\site-packages\psycopg2\__init__.py", line 122, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not translate host name "foreignhost" to address: Unknown host

Notice the final line of this error message calls out the hostname directly saying, “Could not translate host name "foreignhost" to address: Unknown host.” This shows us exactly which credential is causing the issue.

Example 2: Database Connection Request

Alternatively, a correct hostname and an incorrect password is entered.

conn = psycopg2.connect("dbname='postgres' user='postgres' host='localhost' password='badw0rd'")
cur = conn.cursor()

The hostname is correct, but the error now shows badw0rd instead of pa55w0rd. Again, you’ll get an error, but the console message will have changed slightly.

An Output of Example 2

C:\code\Python\PostreSQL\venv\3K\Scripts\python.exe C:/code/python/PostreSQL/unique_violation
Traceback (most recent call last):
  File "C:/code/python/PostreSQL/unique_violation", line 12, in <module>
    conn = psycopg2.connect("dbname='postgres' user='postgres' host='foreignhost' password='pa55w0rd'")
  File "C:\code\Python\PostreSQL\venv\3K\lib\site-packages\psycopg2\__init__.py", line 122, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: connection to server at "localhost" (::1), port 5432 failed: FATAL:  password authentication failed for user "postgres"

The final line of the error calls out the exact source of the error, stating password authentication failed for user "postgres".It is also worth noting that if there are multiple issues, for example, if both the hostname and the password are incorrect, the error will only highlight the first mistake it encounters. In this example, if the hostname were incorrect, it would not be able to attempt validating the user, so the password issue would not be found until the host name was corrected.

How to Prevent An OperationalError in Python

If you are experiencing an OperationalError with a newly written piece of code, the most likely explanation is that the server details entered were incorrect, either due to a typo or inaccurate information being supplied. If this error begins appearing on a code segment which previously worked, it may be that the credentials for the server have been changed. In either case, the console error should call out the exact credential which is causing the issue. This credential can then be confirmed with the database administrator and corrected.

In the event that all the information is correct and the error is still being thrown, this may be evidence of a larger issue in the database server itself, which will need to be troubleshooted by the administrator of the server.

How to Fix OperationalErrors

In most cases, operational errors are straightforward to fix. By observing the line number of the error in the traceback and the exact error text provided, you should be able to isolate the error source and correct it directly. If issues persist, it is best to reach out to the server administrator as the issue is likely coming from the server rather than the Python code.

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