Basic query for Python code¶
Learn to write and run a simple CodeQL query using LGTM.
About the query¶
The query we’re going to run performs a basic search of the code for if
statements that are redundant, in the sense that they only include a pass
statement. For example, code such as:
if error: pass
Running the query¶
In the main search box on LGTM.com, search for the project you want to query. For tips, see Searching.
Click the project in the search results.
Click Query this project.
This opens the query console. (For information about using this, see Using the query console.)
Note
Alternatively, you can go straight to the query console by clicking Query console (at the top of any page), selecting Python from the Language drop-down list, then choosing one or more projects to query from those displayed in the Project drop-down list.
Copy the following query into the text box in the query console:
import python from If ifstmt, Stmt pass where pass = ifstmt.getStmt(0) and pass instanceof Pass select ifstmt, "This 'if' statement is redundant."
LGTM checks whether your query compiles and, if all is well, the Run button changes to green to indicate that you can go ahead and run the query.
Click Run.
The name of the project you are querying, and the ID of the most recently analyzed commit to the project, are listed below the query box. To the right of this is an icon that indicates the progress of the query operation:
Note
Your query is always run against the most recently analyzed commit to the selected project.
The query will take a few moments to return results. When the query completes, the results are displayed below the project name. The query results are listed in two columns, corresponding to the two expressions in the
select
clause of the query. The first column corresponds to the expressionifstmt
and is linked to the location in the source code of the project whereifstmt
occurs. The second column is the alert message.Note
An ellipsis (…) at the bottom of the table indicates that the entire list is not displayed—click it to show more results.
If any matching code is found, click a link in the
ifstmt
column to view theif
statement in the code viewer.The matching
if
statement is highlighted with a yellow background in the code viewer. If any code in the file also matches a query from the standard query library for that language, you will see a red alert message at the appropriate point within the code.
About the query structure¶
After the initial import
statement, this simple query comprises three parts that serve similar purposes to the FROM, WHERE, and SELECT parts of an SQL query.
Query part | Purpose | Details |
---|---|---|
import python |
Imports the standard CodeQL libraries for Python. | Every query begins with one or more import statements. |
from If ifstmt, Stmt pass |
Defines the variables for the query.
Declarations are of the form:
<type> <variable name> |
We use:
|
where pass = ifstmt.getStmt(0) and pass instanceof Pass |
Defines a condition on the variables. |
In other words, the first statement contained in the |
select ifstmt, "This 'if' statement is redundant." |
Defines what to report for each match.
|
Reports the resulting if statement with a string that explains the problem. |
Extend the query¶
Query writing is an inherently iterative process. You write a simple query and then, when you run it, you discover examples that you had not previously considered, or opportunities for improvement.
Remove false positive results¶
Browsing the results of our basic query shows that it could be improved. Among the results you are likely to find examples of if
statements with an else
branch, where a pass
statement does serve a purpose. For example:
if cond():
pass
else:
do_something()
In this case, identifying the if
statement with the pass
statement as redundant is a false positive. One solution to this is to modify the query to ignore pass
statements if the if
statement has an else
branch.
To exclude if
statements that have an else
branch:
Extend the
where
clause to include the following extra condition:and not exists(ifstmt.getOrelse())
The
where
clause is now:where pass = ifstmt.getStmt(0) and pass instanceof Pass and not exists(ifstmt.getOrelse())
Click Run.
There are now fewer results because
if
statements with anelse
branch are no longer included.