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 * @name Empty except
 * @description Except doesn't do anything and has no comment
 * @kind problem
 * @tags quality
 *       reliability
 *       error-handling
 *       external/cwe/cwe-390
 * @problem.severity recommendation
 * @sub-severity high
 * @precision high
 * @id py/empty-except
import python
import semmle.python.ApiGraphs
predicate empty_except(ExceptStmt ex) {
  not exists(Stmt s | s = ex.getAStmt() and not s instanceof Pass)
predicate no_else(ExceptStmt ex) { not exists(ex.getTry().getOrelse()) }
predicate no_comment(ExceptStmt ex) {
  not exists(Comment c |
    c.getLocation().getFile() = ex.getLocation().getFile() and
    c.getLocation().getStartLine() >= ex.getLocation().getStartLine() and
    c.getLocation().getEndLine() <= ex.getBody().getLastItem().getLocation().getEndLine()
predicate non_local_control_flow(ExceptStmt ex) {
  ex.getType() = API::builtin("StopIteration").getAValueReachableFromSource().asExpr()
predicate try_has_normal_exit(Try try) {
  exists(ControlFlowNode pred, ControlFlowNode succ |
    /* Exists a non-exception predecessor, successor pair */
    pred.getASuccessor() = succ and
    not pred.getAnExceptionalSuccessor() = succ
    /* Successor is either a normal flow node or a fall-through exit */
    not exists(Scope s | s.getReturnNode() = succ) and
    /* Predecessor is in try body and successor is not */
    pred.getNode().getParentNode*() = try.getAStmt() and
    not succ.getNode().getParentNode*() = try.getAStmt()
predicate attribute_access(Stmt s) {
  s.(ExprStmt).getValue() instanceof Attribute
  exists(string name | s.(ExprStmt).getValue().(Call).getFunc().(Name).getId() = name |
    name = "getattr" or name = "setattr" or name = "delattr"
  s.(Delete).getATarget() instanceof Attribute
predicate subscript(Stmt s) {
  s.(ExprStmt).getValue() instanceof Subscript
  s.(Delete).getATarget() instanceof Subscript
predicate encode_decode(Call ex, Expr type) {
  exists(string name | ex.getFunc().(Attribute).getName() = name |
    name = "encode" and
    type = API::builtin("UnicodeEncodeError").getAValueReachableFromSource().asExpr()
    name = "decode" and
    type = API::builtin("UnicodeDecodeError").getAValueReachableFromSource().asExpr()
predicate small_handler(ExceptStmt ex, Stmt s, Expr type) {
  not exists(ex.getTry().getStmt(1)) and
  s = ex.getTry().getStmt(0) and
  ex.getType() = type
predicate focussed_handler(ExceptStmt ex) {
  exists(Stmt s, Expr type | small_handler(ex, s, type) |
    subscript(s) and
    type = API::builtin("IndexError").getASubclass*().getAValueReachableFromSource().asExpr()
    attribute_access(s) and
    type = API::builtin("AttributeError").getAValueReachableFromSource().asExpr()
    s.(ExprStmt).getValue() instanceof Name and
    type = API::builtin("NameError").getAValueReachableFromSource().asExpr()
    encode_decode(s.(ExprStmt).getValue(), type)
Try try_return() { not exists(result.getStmt(1)) and result.getStmt(0) instanceof Return }
from ExceptStmt ex
  empty_except(ex) and
  no_else(ex) and
  no_comment(ex) and
  not non_local_control_flow(ex) and
  not ex.getTry() = try_return() and
  try_has_normal_exit(ex.getTry()) and
  not focussed_handler(ex)
select ex, "'except' clause does nothing but pass and there is no explanatory comment."

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