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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

where

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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