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pychecker - program to check python scripts for common mistakes


pychecker [options] files...


This manual page documents briefly the pychecker command. It was written for the Debian GNU/Linux distribution because the original program does not have a manual page.

pychecker is a program that finds common bugs in python source code. It finds problems that are typically caught by a compiler for less dynamic languages, like C and C++. Because of the dynamic nature of python, some warnings may be incorrect; however, spurious warnings should be fairly infrequent.

pychecker works in a combination of ways. First, it imports each module. If there is an import error, the module cannot be processed. The import provides some basic information about the module. The code for each function, class, and method is checked for possible problems.


By default, the /usr/bin/pychecker script executes pychecker using the default Python interpreter in /usr/bin/python. Some users may want to run pychecker with a different Python interpreter. To accommodate this, the
/usr/bin/pychecker script supports the use of $PYTHONVER on the command-line, i.e.

If $PYTHONVER is set, /usr/bin/pychecker will attempt to execute pychecker using /usr/bin/python$PYTHONVER instead of /usr/bin/python.

Note, however, that this will only work for versions of the python interpreter supported by the current Debian Python infrastructure, not for any arbitrary Python interpreter installed in /usr/bin.


Long options can be preceded with no- to turn off (e.g., no-namedargs). Default settings are indicated by [brackets]

Major Options:

only warn about files passed on the command line [off]
the maximum error level of warnings to be displayed
-#, --limit
the maximum number of warnings to be displayed [10]
specify .pycheckrc file to use
support Quixote's PTL modules
list of evil C extensions that crash the interpreter [[]]

Error Control:

unused imports [on]
unused imports from [on]

-M, --reimportself module imports itself [on]

reimporting a module [on]
module does import and from ... import [on]
unused local variables, except tuples [on]
all unused local variables, including tuples [off]
-9, --members
all unused class data members [off]
all unused module variables [off]
unused private module variables [on]
report each occurrence of global warnings [off]
functions called with named arguments (like keywords) [off]
Attributes (members) must be defined in __init__() [off]

-I, --initsubclass Subclass.__init__() not defined [off]

Baseclass.__init__() not called [on]
-0, --abstract
Subclass needs to override methods that only throw exceptions [on]
Return None from __init__() [on]
-8, --unreachable
unreachable code [off]
-2, --constCond
a constant is used in a conditional statement [on]
-1, --constant1
1 is used in a conditional statement (if 1: or while 1:) [off]
check if iterating over a string [on]
check improper use of string.find() [on]
Calling data members as functions [off]
class attribute does not exist [on]
First argument to methods [self]

--classmethodargs First argument to classmethods [['cls', 'klass']]

unused method/function arguments [on]
unused method/function variable arguments [on]
ignore if self is unused in methods [off]
check if overridden methods have the same signature [on]
check if __special__ methods exist and have the correct signature [on]
check if function/class/method names are reused [on]
check if using unary positive (+) which is usually meaningless [on]
check if modify (call method) on a parameter that has a default value [on]
check if variables are set to different types [off]
check if unpacking a non-sequence [on]
check if unpacking sequence with the wrong length [on]
check if raising or catching bad exceptions [on]
-4, --noeffect
check if statement appears to have no effect [on]
check if using (expr % 1), it has no effect on integers and strings [on]
check if using (expr is const-literal), doesn't always work on integers and strings [on]

Possible Errors:

-r, --returnvalues check consistent return values [on]

-C, --implicitreturns check if using implict and explicit return values [on]

check that attributes of objects exist [on]
-7, --slots
various warnings about incorrect usage of __slots__ [on]
-3, --properties
using properties with classic classes [on]
check if __slots__ is empty [on]
check if using integer division [on]
check if local variable shadows a global [on]

-s, --shadowbuiltin check if a variable shadows a builtin [on]


check if input() is used [on]
-6, --exec
check if the exec statement is used [off]


ignore warnings from files under standard library [off]
ignore warnings from the list of modules [['Tkinter', 'wxPython', 'gtk', 'GTK', 'GDK']]
ignore global variables not used if name is one of these values [['__version__', '__warningregistry__', '__all__', '__credits__', '__test__', '__author__', '__email__', '__revision__', '__id__', '__copyright__', '__license__', '__date__']]
ignore unused locals/arguments if name is one of these values [['_', 'empty', 'unused', 'dummy']]
ignore use of deprecated modules/functions [on]


maximum lines in a function [200]
maximum branches in a function [50]
maximum returns in a function [10]
maximum # of arguments to a function [10]
maximum # of locals in a function [40]
-5, --maxrefs
maximum # of identifier references (Law of Demeter) [5]
no module doc strings [off]
no class doc strings [off]
no function/method doc strings [off]


print a .pycheckrc file generated from command line args
print internal checker parse structures [off]
turn on debugging for checker [off]
turn off all output except warnings [off]
print the version of PyChecker and exit


Pychecker was written by Neal Norwitz and Eric C. Newton. Other contributors are listed in /usr/share/doc/pychecker/MAINTAINERS

This manual page was written by Fredrik Steen <> for the Debian GNU/Linux system (but may be used by others). It has been subsequently modified by Kenneth J. Pronovici <>.



Running pychecker --help may provide more complete documentation of program options. The pychecker web page at <> contains some guidance on using the program.

Jan 2011 pychecker 0.8.19