PyChecker: Finding Bugs in Other People's Programs

PyChecker: Finding Bugs in Other People's Programs

Neal Norwitz, Eric C. Newton, Michele Moore

MetaSlash, Inc.


PyChecker is a software development tool used to find programming errors in Python code. Like a compiler, it performs the static analysis used by less dynamic languages like C/C++ and Java. Errors are found by examining the structural elements of the code, such as modules, functions, classes, and executable statements. This paper describes the motivations, design, and benefits of PyChecker.

Keywords: code inspection, static analysis, debugging tool


PyChecker [1] began as an experiment to determine the feasibility of using static analysis on Python [2] programs. Python is a highly dynamic language that does not lend itself to the static analysis typically performed by compilers. Some people in the computer science community believe the development of robust and/or complex software requires compile-time checks for software correctness. Although compilers detect many bugs, they do not eliminate all bugs. Furthermore, a common misconception is that scripting languages like Python are less robust than compiled languages and should not be used for mission critical systems.

In the beginning, PyChecker only found missing doc strings. This technique provided the learning necessary to make more advanced checks. Missing and unused global checks were later added. Soon, we had built up enough internal information about methods and functions to check for the proper number of arguments in function calls.

We have worked on mission critical, high availability, air traffic control software and Python is a great language for many of these systems. However, development with Python can be improved to help software developers reduce bugs. PyChecker was created in an effort to find bugs during development prior to testing or delivering our systems.

The initial release of PyChecker demonstrated it could find bugs by static analysis. The positive feedback from many developers proved the value of PyChecker. With added incentive, we implemented more features to find other types of bugs.


Motivation for Developing PyChecker

We find ourselves using Python in a broad range of systems that are being extended and maintained for many years. Python is an excellent language for these applications, because it helps avoid such runtime conditions as out-of-bounds errors and memory leaks.

Developing large, robust systems requires significant testing; all paths through the code must be executed to achieve full test coverage. PyChecker supports the development of these systems by automating code inspection and checking for errors in ways that testing does not. Generally, exceptional conditions and error handling code are tested the least because they are rarely, if ever, executed. Therefore, undetected errors are more likely and handling an unexpected event can become a catastrophic error.

When making changes to complex systems, modifications must be done carefully, backed by large investments in testing. However, it's not always possible to test all aspects of a system. Less dynamic languages like C/C++ and Java check for errors during the static analysis phase of compilation. These languages require type declarations, for example, to find coding errors related to mistyped variables or parameters. Other types of bugs can be found in this first pass of code inspection, like calling a function with the wrong number of parameters or forgetting to import a module. However, Python finds these bugs later, during execution of the program, because it doesn't perform static analysis.

Static Analysis

Static analysis is not necessary for a language to be useful or even powerful. We love Python -- it is clean, portable, and easy-to-use. However, it is also easy to write code in Python and make an undetected mistake. It is even easier to refactor working code and make a mistake. We encountered many instances where common problems could be found through code inspection. PyChecker was developed to do the inspection automatically, so we could develop more robust systems.

Static analysis has been around a long time. lint [3] has been used for decades on C code. gcc [4] has many options to configure warnings, including -Wall to turn on all common warnings. perl [5] has the use strict; and use warnings; directives as well as -w.

More recently the Stanford Validity Checker [6] has been used to find hundreds of semantic bugs in the Linux kernel. However, Python had no equivalent to these tools. There is a tool to freeze code for a specific architecture so that the Python interpreter does not need to be installed. This process will not find invalid references, such as a call to a function that does not exist. While static analysis has been considered for Python [7], this work has not yet been used to develop PyChecker.

What PyChecker Does

Currently PyChecker produces four different categories of warnings:

Likely Bug WarningsPotential Bug WarningsUnused Identifier WarningsCode Complexity/Style Warnings

Potential security warnings will be implemented in the near future. Many other enhancements are also planned, for example: more type checking, more checking for code that can throw exceptions, path analysis, and unreachable code.

To get a better idea of the types of errors PyChecker can detect, examine the following code example for errors: (Hint: there are 14!)

 1   #! /usr/bin/env python
 3   'Example errors caught by PyChecker'
 5   import string
 7   metaslash = 1
 9   def printNames():
10       neal = 'neal'
11       michelle = 'michele'
12       eric = 5
13       print "Local values: %(neal)S %(michele)s %(eric)" % locals()
15   class Nothing:
16       def printValue(value):
17           print value
18       def set(self, value):
19           self.value = value
21   def tryToDoSomething(self, value):
22       try:
23           import string
24           if not value:
25               raise RuntimeError, "Hey, there's no value"
26           printNames('a, b, c')
27       except:
28           traceback.print_exc()
30   def setGlobal(value=None):
31       print 'Old MetaSlash value is:', metaslash
32       metaslash = value
33       useless = Nothing(5)
34       print 'a useless value is:', useless.valeu

With default options, PyChecker produces: Imported module (string) not used Local variable (michelle) not used Invalid format string, problem starts near: '(eric)' Invalid format string, problem starts near: '(neal)S ' No local variable (michele) self is not first method argument self is argument in function Local variable (string) not used Module (string) re-imported Invalid arguments to (printNames), got 1, expected 0 No global (traceback) found Variable (metaslash) used before being set Instantiating an object with arguments, but no constructor Object (useless) has no attribute (valeu)

Because programmers have different coding styles, PyChecker needs to be adaptable. Warnings can be enabled or disabled so that only the desired warnings are produced. For example, the "unused variable" warning can be disabled since it is not necessarily an error. There are four different ways to customize PyChecker's configuration:

  1. Use command line parameters to change the default behavior
  2. Define configuration values in a .pycheckrc file
  3. Set the variable __pychecker__ in code
  4. Define a suppression dictionary (in .pycheckrc file) where the keys are: 'module', 'module.function', 'module.class', or 'module.class.method' and the values use the same format as the __pychecker__ variable.

By providing a flexible way to suppress warnings, spurious warnings can be minimized. The code complexity and style warnings can be adjusted to match each user's preference. In addition, a "blacklist" can be specified as a list of module names for which warnings should not be produced. Generally, libraries are blacklisted because their code doesn't change, therefore, neither do their warnings.

How PyChecker Works

PyChecker imports all the modules passed on the command line. As it imports each module, PyChecker determines the attributes of the module (imported modules, classes, and functions) using dir(). It creates a data tree of all the modules, functions, global variables, classes, and methods using type(). During this pass, information about function and method signatures is gathered. After generating the data tree from the code, PyChecker iterates through the Python byte code. Byte codes are the equivalent of machine instructions in the Python interpreter and are generated by the interpreter when a module is imported. After iterating through all the byte codes, the final set of warnings is produced.

Byte codes are a convenient format to use for finding errors because the data for each operation is readily available. While iterating through the byte code, warnings are generated by:

To better understand the Python byte codes, consider the following function contained in a file

def increment(x): return x + 1

The following interactive session with the Python interpreter shows the function's disassembled byte code (dis is part of the standard library):

>>> import example1
>>> import dis
>>> dis.dis(example1.increment)
  0 SET_LINENO               1

  3 SET_LINENO               1
  6 LOAD_FAST                0 (x)
  9 LOAD_CONST               1 (1)
 14 LOAD_CONST               0 (None)

For each byte code instruction, a function is dispatched based upon the op-code. For example, the op-code SET_LINENO is dispatched to a function which saves the current line number. Some of the dispatch functions construct objects for later analysis. One of the primary sources of information is the inferred status of the stack. PyChecker recovers a significant part of program flow by analyzing the stack operations. Many of the dispatch functions, like LOAD_FAST, build an internal representation of what the stack will do at runtime. From this internal representation, PyChecker can determine things like the number of arguments passed to a function and the initial type of local constants.

A C programmer would expect the following code to increment the value stored in x. Although perfectly legal in Python, this function actually has no effect on x:

def bad_increment(x): return ++x

This code produces the following the byte codes:

>>> import example2
>>> import dis
>>> dis.dis(example2.bad_increment)
  0 SET_LINENO               1

  3 SET_LINENO               1
  6 LOAD_FAST                0 (x)
 12 LOAD_CONST               0 (None)

Now it is easier to see why ++x is a statement with no effect. ++ is not auto-increment, rather it is +(+x). The Python interpreter does not flag this as an error because the code is syntactically correct. The same problem exists with --x or ~~x, in that these statements typically have no effect.

Python byte code is easy to use and manipulate; however, there are several drawbacks with the current techniques employed by PyChecker:

Future releases of PyChecker are expected to address these issues.

Development Approach

PyChecker was released in April 2001 with the ability to produce 18 warnings. Most warnings were fairly simple and only Python 2.0 was supported. Seven months and sixteen releases later, PyChecker is at version 0.8.6, reports 63 warnings, and supports all versions of Python from 1.5.2 and later.

The original warnings produced were fairly simplistic. PyChecker also produced some inappropriate warnings because not all language features were supported. The releases were generally quick, often at two-week intervals. New features and configuration options were added with each release as well as a few bug fixes. As we learned more about Python, more checks were implemented which sometimes resulted in bugs, but often resulted in more ideas for additional checks to add. With a highly iterative approach (build--test--release), the system progressed much faster than many other projects we have worked on. Features were added and bugs were fixed quickly and easily.

With all of the development of PyChecker, refactoring was necessary from time to time. PyChecker has been a big help with finding bugs in itself! Refactoring commonly entails moving blocks of code between functions, methods, and modules, resulting in missing imports or extra imports. Members and methods that existed in the old class no longer exist in the new class. PyChecker would ferret out these problems very quickly. Rather than require a test run, it could quickly inspect the code and report problems.

PyChecker is not a substitute for testing; on the contrary, it augments testing. During development and prior to release, PyChecker is run on itself to ensure that it doesn't produce false warnings. There are over 50 unit tests which are also run. Each bug report generates a test case so that errors are not duplicated in the future. With the combination of running PyChecker and unit testing, we have been able to decrease the bugs in our systems.


Many bugs can be found by static analysis tools. Such tools complement testing and should be used by developers to create more robust software. PyChecker is a real-world example of how beneficial static analysis is throughout the development process.

PyChecker produced a number of surprising results:

There is no shortage of bugs PyChecker can find. In the future, PyChecker will provide better static analysis to provide warnings for all object access, return values, security holes, unreachable code, etc. If you have any suggestions, contact


[1] PyChecker -
[2] Python -
[3] "Lint, a C Program Checker", Computer Science Technical Report 65, Bell Labs, Murray Hill, NJ. updated version TM 78-1273-8
man page -
[4] GCC (GNU Compiler Collection) -
[5] Perl -
[6] "Checking System Rules Using System-Specific, Programmer-Written Compiler Extensions" (Stanford Validity Checker), Engler, Dawson, et al.,
[7] "Aggressive Type Inference", Aycock, John,