CptS 481 - Python Software Construction

Unit 13: Checking and Debugging

from IPython.display import HTML
HTML(open("../include/notes.css", "r").read())

pylint

pylint takes a "picky" approach to code analysis. The "lint" part of the name comes from an early C program checker.

We start with the same "0_sloppy.py" file:

%%file 0_sloppy.py
import time

class Aclass:

    def __init__(self):
        return None
    
    def aMethod(self, var1, var2):
        self.var3 = var1*var2

    def bMethod(self):
        '''
        A simple function
        '''
        pass
        
def main():
    int = 10
    time = 'today'
    aObj = Aclass()
    aObj.var3 = 0
    val1 = 13.0
    flag = 0    
    if (int == 10):
        flagg = 1

    if (flag):
        print('Flag has been set: ')

    aObj.afunc(int, val1)
    print(aObj.var3)
    
main()

Overwriting 0_sloppy.py

%%sh
python3 -m pylint demos/0_sloppy.py || exit 0 # make Notebook ignore error status

************* Module 0_sloppy
demos/0_sloppy.py:7:0: C0303: Trailing whitespace (trailing-whitespace)
demos/0_sloppy.py:17:12: C0303: Trailing whitespace (trailing-whitespace)
demos/0_sloppy.py:18:0: C0325: Unnecessary parens after 'if' keyword (superfluous-parens)
demos/0_sloppy.py:21:0: C0325: Unnecessary parens after 'if' keyword (superfluous-parens)
demos/0_sloppy.py:26:0: C0303: Trailing whitespace (trailing-whitespace)
demos/0_sloppy.py:1:0: C0103: Module name "0_sloppy" doesn't conform to snake_case naming style (invalid-name)
demos/0_sloppy.py:1:0: C0114: Missing module docstring (missing-module-docstring)
demos/0_sloppy.py:3:0: C0115: Missing class docstring (missing-class-docstring)
demos/0_sloppy.py:8:4: C0116: Missing function or method docstring (missing-function-docstring)
demos/0_sloppy.py:3:0: R0903: Too few public methods (1/2) (too-few-public-methods)
demos/0_sloppy.py:9:8: W0201: Attribute 'var3' defined outside __init__ (attribute-defined-outside-init)
demos/0_sloppy.py:15:4: W0201: Attribute 'var3' defined outside __init__ (attribute-defined-outside-init)
demos/0_sloppy.py:12:4: W0622: Redefining built-in 'int' (redefined-builtin)
demos/0_sloppy.py:13:4: W0621: Redefining name 'time' from outer scope (line 1) (redefined-outer-name)
demos/0_sloppy.py:11:0: C0116: Missing function or method docstring (missing-function-docstring)
demos/0_sloppy.py:14:4: C0103: Variable name "aObj" doesn't conform to snake_case naming style (invalid-name)
demos/0_sloppy.py:13:4: W0612: Unused variable 'time' (unused-variable)
demos/0_sloppy.py:19:8: W0612: Unused variable 'flagg' (unused-variable)
demos/0_sloppy.py:1:0: W0611: Unused import time (unused-import)

------------------------------------------------------------------
Your code has been rated at 0.50/10 (previous run: 0.50/10, +0.00)

So if we clean this up a bit...

%%file demos/2_pylint_tidy.py
"""
small script containing some potential bugs
"""
#pylint: disable=C0103

__pychecker__ = 'initattr classdoc funcdoc'

class Aclass:
    """
    This is a class.
    """
    def __init__(self):
        self.var3 = None

    def afunc(self, var1, var2):
        '''
        A simple function
        '''
        self.var3 = var1*var2

    def bfunc(self):
        '''
        A simple function
        '''
        pass

def main():
    '''
    The main function
    '''
    _int = 10
    aObj = Aclass()
    aObj.var3 = 0
    val1 = 13.0
    flag = 0
    if int == 10:
        flag = 1

    if flag:
        print('Flag has been set: ')

    aObj.afunc(_int, val1)
    print(aObj.var3)

main()

Overwriting demos/2_pylint_tidy.py

%%sh
pylint demos/2_pylint_tidy.py || exit 0 # make Notebook ignore error status

************* Module 2_pylint_tidy
demos/2_pylint_tidy.py:25:8: W0107: Unnecessary pass statement (unnecessary-pass)

------------------------------------------------------------------
Your code has been rated at 9.52/10 (previous run: 9.52/10, +0.00)

Let's try a more useful program: "3_polar_for_sphinx.py".

"""
The ``polar`` Module
"""
from math import sin, cos, atan2, pi

__version__ = "1.0"

class Polar:
    """
    specifies a 2D position or velocity in polar coordinatesThes

    This class represents a 2D position or velocity measured in polar
    coordinates and defined by its `distance` (or speed) and an angle
    (`bearing`) (in radians) from the positive y ('North') direction.

    We follow the conventions of navigation, not mathematics, so 0
    degrees is North and bearings increase in a clockwise fashion.
    """

    DEGREES_TO_RADIANS = pi / 180
    "multiplicative constant to convert degrees to radians"

    #: multiplicative constant to convert radians to degrees
    RADIANS_TO_DEGREES = 180 / pi

    def __init__(self, distance=0.0, bearing=0.0):
        """
        specifies a polar point by `distance` and `bearing`
        """
        self.distance = distance
        self.bearing = bearing

    def rect(self):
        """
        returns the rectangular (Cartesian) coords as an (x, y) tuple
        """
        return (self.distance*sin(self.bearing),
                self.distance*cos(self.bearing))

    def __add__(self, other):
        """
        returns the result of adding two `Polar`s
        """
        (x0, y0) = self.rect()
        (x1, y1) = other.rect()
        dx = x1 + x0
        dy = y1 + y0
        mag = (dx**2 + dy**2)**0.5
        return Polar(mag, atan2(dy, dx) if mag > 0 else 0)

    def __repr__(self):
        """
        returns a 'lossless' string representation of a PolarCoordinate
        """
        return "Polar({}, {})".format(self.distance, self.bearing)


if __name__ == '__main__':
    # test1: with a 3-4-5 right triangle
    a = Polar(3, 0)
    print('  a:', a)
    b = Polar(4, pi/2)
    print('  b:', b)
    print('a+b:', a+b)

    # test2: zero relative distance
    c = Polar(2, Polar.DEGREES_TO_RADIANS * 30)
    print('  c:', c)
    d = Polar(2, Polar.DEGREES_TO_RADIANS * 210)
    print('  d:', d)
    print('c+d:', c+d)

  a: Polar(3, 0)
  b: Polar(4, 1.5707963267948966)
a+b: Polar(5.0, 0.6435011087932845)
  c: Polar(2, 0.5235987755982988)
  d: Polar(2, 3.6651914291880923)
c+d: Polar(4.002966042486721e-16, 2.5535900500422257)

%%sh
pylint demos/3_polar_for_sphinx.py || exit 0 # make Notebook ignore error status

************* Module 3_polar_for_sphinx
demos/3_polar_for_sphinx.py:1:0: C0103: Module name "3_polar_for_sphinx" doesn't conform to snake_case naming style (invalid-name)
demos/3_polar_for_sphinx.py:44:9: C0103: Variable name "x0" doesn't conform to snake_case naming style (invalid-name)
demos/3_polar_for_sphinx.py:44:13: C0103: Variable name "y0" doesn't conform to snake_case naming style (invalid-name)
demos/3_polar_for_sphinx.py:45:9: C0103: Variable name "x1" doesn't conform to snake_case naming style (invalid-name)
demos/3_polar_for_sphinx.py:45:13: C0103: Variable name "y1" doesn't conform to snake_case naming style (invalid-name)
demos/3_polar_for_sphinx.py:46:8: C0103: Variable name "dx" doesn't conform to snake_case naming style (invalid-name)
demos/3_polar_for_sphinx.py:47:8: C0103: Variable name "dy" doesn't conform to snake_case naming style (invalid-name)

------------------------------------------------------------------
Your code has been rated at 7.74/10 (previous run: 7.74/10, +0.00)

%%sh
pylint --generate-rcfile

[MASTER]

# A comma-separated list of package or module names from where C extensions may
# be loaded. Extensions are loading into the active Python interpreter and may
# run arbitrary code.
extension-pkg-whitelist=

# Specify a score threshold to be exceeded before program exits with error.
fail-under=10.0

# Add files or directories to the blacklist. They should be base names, not
# paths.
ignore=CVS

# Add files or directories matching the regex patterns to the blacklist. The
# regex matches against base names, not paths.
ignore-patterns=

# Python code to execute, usually for sys.path manipulation such as
# pygtk.require().
#init-hook=

# Use multiple processes to speed up Pylint. Specifying 0 will auto-detect the
# number of processors available to use.
jobs=1

# Control the amount of potential inferred values when inferring a single
# object. This can help the performance when dealing with large functions or
# complex, nested conditions.
limit-inference-results=100

# List of plugins (as comma separated values of python module names) to load,
# usually to register additional checkers.
load-plugins=

# Pickle collected data for later comparisons.
persistent=yes

# When enabled, pylint would attempt to guess common misconfiguration and emit
# user-friendly hints instead of false-positive error messages.
suggestion-mode=yes

# Allow loading of arbitrary C extensions. Extensions are imported into the
# active Python interpreter and may run arbitrary code.
unsafe-load-any-extension=no


[MESSAGES CONTROL]

# Only show warnings with the listed confidence levels. Leave empty to show
# all. Valid levels: HIGH, INFERENCE, INFERENCE_FAILURE, UNDEFINED.
confidence=

# Disable the message, report, category or checker with the given id(s). You
# can either give multiple identifiers separated by comma (,) or put this
# option multiple times (only on the command line, not in the configuration
# file where it should appear only once). You can also use "--disable=all" to
# disable everything first and then reenable specific checks. For example, if
# you want to run only the similarities checker, you can use "--disable=all
# --enable=similarities". If you want to run only the classes checker, but have
# no Warning level messages displayed, use "--disable=all --enable=classes
# --disable=W".
disable=print-statement,
        parameter-unpacking,
        unpacking-in-except,
        old-raise-syntax,
        backtick,
        long-suffix,
        old-ne-operator,
        old-octal-literal,
        import-star-module-level,
        non-ascii-bytes-literal,
        raw-checker-failed,
        bad-inline-option,
        locally-disabled,
        file-ignored,
        suppressed-message,
        useless-suppression,
        deprecated-pragma,
        use-symbolic-message-instead,
        apply-builtin,
        basestring-builtin,
        buffer-builtin,
        cmp-builtin,
        coerce-builtin,
        execfile-builtin,
        file-builtin,
        long-builtin,
        raw_input-builtin,
        reduce-builtin,
        standarderror-builtin,
        unicode-builtin,
        xrange-builtin,
        coerce-method,
        delslice-method,
        getslice-method,
        setslice-method,
        no-absolute-import,
        old-division,
        dict-iter-method,
        dict-view-method,
        next-method-called,
        metaclass-assignment,
        indexing-exception,
        raising-string,
        reload-builtin,
        oct-method,
        hex-method,
        nonzero-method,
        cmp-method,
        input-builtin,
        round-builtin,
        intern-builtin,
        unichr-builtin,
        map-builtin-not-iterating,
        zip-builtin-not-iterating,
        range-builtin-not-iterating,
        filter-builtin-not-iterating,
        using-cmp-argument,
        eq-without-hash,
        div-method,
        idiv-method,
        rdiv-method,
        exception-message-attribute,
        invalid-str-codec,
        sys-max-int,
        bad-python3-import,
        deprecated-string-function,
        deprecated-str-translate-call,
        deprecated-itertools-function,
        deprecated-types-field,
        next-method-defined,
        dict-items-not-iterating,
        dict-keys-not-iterating,
        dict-values-not-iterating,
        deprecated-operator-function,
        deprecated-urllib-function,
        xreadlines-attribute,
        deprecated-sys-function,
        exception-escape,
        comprehension-escape

# Enable the message, report, category or checker with the given id(s). You can
# either give multiple identifier separated by comma (,) or put this option
# multiple time (only on the command line, not in the configuration file where
# it should appear only once). See also the "--disable" option for examples.
enable=c-extension-no-member


[REPORTS]

# Python expression which should return a score less than or equal to 10. You
# have access to the variables 'error', 'warning', 'refactor', and 'convention'
# which contain the number of messages in each category, as well as 'statement'
# which is the total number of statements analyzed. This score is used by the
# global evaluation report (RP0004).
evaluation=10.0 - ((float(5 * error + warning + refactor + convention) / statement) * 10)

# Template used to display messages. This is a python new-style format string
# used to format the message information. See doc for all details.
#msg-template=

# Set the output format. Available formats are text, parseable, colorized, json
# and msvs (visual studio). You can also give a reporter class, e.g.
# mypackage.mymodule.MyReporterClass.
output-format=text

# Tells whether to display a full report or only the messages.
reports=no

# Activate the evaluation score.
score=yes


[REFACTORING]

# Maximum number of nested blocks for function / method body
max-nested-blocks=5

# Complete name of functions that never returns. When checking for
# inconsistent-return-statements if a never returning function is called then
# it will be considered as an explicit return statement and no message will be
# printed.
never-returning-functions=sys.exit


[TYPECHECK]

# List of decorators that produce context managers, such as
# contextlib.contextmanager. Add to this list to register other decorators that
# produce valid context managers.
contextmanager-decorators=contextlib.contextmanager

# List of members which are set dynamically and missed by pylint inference
# system, and so shouldn't trigger E1101 when accessed. Python regular
# expressions are accepted.
generated-members=

# Tells whether missing members accessed in mixin class should be ignored. A
# mixin class is detected if its name ends with "mixin" (case insensitive).
ignore-mixin-members=yes

# Tells whether to warn about missing members when the owner of the attribute
# is inferred to be None.
ignore-none=yes

# This flag controls whether pylint should warn about no-member and similar
# checks whenever an opaque object is returned when inferring. The inference
# can return multiple potential results while evaluating a Python object, but
# some branches might not be evaluated, which results in partial inference. In
# that case, it might be useful to still emit no-member and other checks for
# the rest of the inferred objects.
ignore-on-opaque-inference=yes

# List of class names for which member attributes should not be checked (useful
# for classes with dynamically set attributes). This supports the use of
# qualified names.
ignored-classes=optparse.Values,thread._local,_thread._local

# List of module names for which member attributes should not be checked
# (useful for modules/projects where namespaces are manipulated during runtime
# and thus existing member attributes cannot be deduced by static analysis). It
# supports qualified module names, as well as Unix pattern matching.
ignored-modules=

# Show a hint with possible names when a member name was not found. The aspect
# of finding the hint is based on edit distance.
missing-member-hint=yes

# The minimum edit distance a name should have in order to be considered a
# similar match for a missing member name.
missing-member-hint-distance=1

# The total number of similar names that should be taken in consideration when
# showing a hint for a missing member.
missing-member-max-choices=1

# List of decorators that change the signature of a decorated function.
signature-mutators=


[VARIABLES]

# List of additional names supposed to be defined in builtins. Remember that
# you should avoid defining new builtins when possible.
additional-builtins=

# Tells whether unused global variables should be treated as a violation.
allow-global-unused-variables=yes

# List of strings which can identify a callback function by name. A callback
# name must start or end with one of those strings.
callbacks=cb_,
          _cb

# A regular expression matching the name of dummy variables (i.e. expected to
# not be used).
dummy-variables-rgx=_+$|(_[a-zA-Z0-9_]*[a-zA-Z0-9]+?$)|dummy|^ignored_|^unused_

# Argument names that match this expression will be ignored. Default to name
# with leading underscore.
ignored-argument-names=_.*|^ignored_|^unused_

# Tells whether we should check for unused import in __init__ files.
init-import=no

# List of qualified module names which can have objects that can redefine
# builtins.
redefining-builtins-modules=six.moves,past.builtins,future.builtins,builtins,io


[SIMILARITIES]

# Ignore comments when computing similarities.
ignore-comments=yes

# Ignore docstrings when computing similarities.
ignore-docstrings=yes

# Ignore imports when computing similarities.
ignore-imports=no

# Minimum lines number of a similarity.
min-similarity-lines=4


[SPELLING]

# Limits count of emitted suggestions for spelling mistakes.
max-spelling-suggestions=4

# Spelling dictionary name. Available dictionaries: none. To make it work,
# install the python-enchant package.
spelling-dict=

# List of comma separated words that should not be checked.
spelling-ignore-words=

# A path to a file that contains the private dictionary; one word per line.
spelling-private-dict-file=

# Tells whether to store unknown words to the private dictionary (see the
# --spelling-private-dict-file option) instead of raising a message.
spelling-store-unknown-words=no


[LOGGING]

# The type of string formatting that logging methods do. `old` means using %
# formatting, `new` is for `{}` formatting.
logging-format-style=old

# Logging modules to check that the string format arguments are in logging
# function parameter format.
logging-modules=logging


[FORMAT]

# Expected format of line ending, e.g. empty (any line ending), LF or CRLF.
expected-line-ending-format=

# Regexp for a line that is allowed to be longer than the limit.
ignore-long-lines=^\s*(# )?<?https?://\S+>?$

# Number of spaces of indent required inside a hanging or continued line.
indent-after-paren=4

# String used as indentation unit. This is usually "    " (4 spaces) or "\t" (1
# tab).
indent-string='    '

# Maximum number of characters on a single line.
max-line-length=100

# Maximum number of lines in a module.
max-module-lines=1000

# Allow the body of a class to be on the same line as the declaration if body
# contains single statement.
single-line-class-stmt=no

# Allow the body of an if to be on the same line as the test if there is no
# else.
single-line-if-stmt=no


[MISCELLANEOUS]

# List of note tags to take in consideration, separated by a comma.
notes=FIXME,
      XXX,
      TODO

# Regular expression of note tags to take in consideration.
#notes-rgx=


[STRING]

# This flag controls whether inconsistent-quotes generates a warning when the
# character used as a quote delimiter is used inconsistently within a module.
check-quote-consistency=no

# This flag controls whether the implicit-str-concat should generate a warning
# on implicit string concatenation in sequences defined over several lines.
check-str-concat-over-line-jumps=no


[BASIC]

# Naming style matching correct argument names.
argument-naming-style=snake_case

# Regular expression matching correct argument names. Overrides argument-
# naming-style.
#argument-rgx=

# Naming style matching correct attribute names.
attr-naming-style=snake_case

# Regular expression matching correct attribute names. Overrides attr-naming-
# style.
#attr-rgx=

# Bad variable names which should always be refused, separated by a comma.
bad-names=foo,
          bar,
          baz,
          toto,
          tutu,
          tata

# Bad variable names regexes, separated by a comma. If names match any regex,
# they will always be refused
bad-names-rgxs=

# Naming style matching correct class attribute names.
class-attribute-naming-style=any

# Regular expression matching correct class attribute names. Overrides class-
# attribute-naming-style.
#class-attribute-rgx=

# Naming style matching correct class names.
class-naming-style=PascalCase

# Regular expression matching correct class names. Overrides class-naming-
# style.
#class-rgx=

# Naming style matching correct constant names.
const-naming-style=UPPER_CASE

# Regular expression matching correct constant names. Overrides const-naming-
# style.
#const-rgx=

# Minimum line length for functions/classes that require docstrings, shorter
# ones are exempt.
docstring-min-length=-1

# Naming style matching correct function names.
function-naming-style=snake_case

# Regular expression matching correct function names. Overrides function-
# naming-style.
#function-rgx=

# Good variable names which should always be accepted, separated by a comma.
good-names=i,
           j,
           k,
           ex,
           Run,
           _

# Good variable names regexes, separated by a comma. If names match any regex,
# they will always be accepted
good-names-rgxs=

# Include a hint for the correct naming format with invalid-name.
include-naming-hint=no

# Naming style matching correct inline iteration names.
inlinevar-naming-style=any

# Regular expression matching correct inline iteration names. Overrides
# inlinevar-naming-style.
#inlinevar-rgx=

# Naming style matching correct method names.
method-naming-style=snake_case

# Regular expression matching correct method names. Overrides method-naming-
# style.
#method-rgx=

# Naming style matching correct module names.
module-naming-style=snake_case

# Regular expression matching correct module names. Overrides module-naming-
# style.
#module-rgx=

# Colon-delimited sets of names that determine each other's naming style when
# the name regexes allow several styles.
name-group=

# Regular expression which should only match function or class names that do
# not require a docstring.
no-docstring-rgx=^_

# List of decorators that produce properties, such as abc.abstractproperty. Add
# to this list to register other decorators that produce valid properties.
# These decorators are taken in consideration only for invalid-name.
property-classes=abc.abstractproperty

# Naming style matching correct variable names.
variable-naming-style=snake_case

# Regular expression matching correct variable names. Overrides variable-
# naming-style.
#variable-rgx=


[DESIGN]

# Maximum number of arguments for function / method.
max-args=5

# Maximum number of attributes for a class (see R0902).
max-attributes=7

# Maximum number of boolean expressions in an if statement (see R0916).
max-bool-expr=5

# Maximum number of branch for function / method body.
max-branches=12

# Maximum number of locals for function / method body.
max-locals=15

# Maximum number of parents for a class (see R0901).
max-parents=7

# Maximum number of public methods for a class (see R0904).
max-public-methods=20

# Maximum number of return / yield for function / method body.
max-returns=6

# Maximum number of statements in function / method body.
max-statements=50

# Minimum number of public methods for a class (see R0903).
min-public-methods=2


[CLASSES]

# List of method names used to declare (i.e. assign) instance attributes.
defining-attr-methods=__init__,
                      __new__,
                      setUp,
                      __post_init__

# List of member names, which should be excluded from the protected access
# warning.
exclude-protected=_asdict,
                  _fields,
                  _replace,
                  _source,
                  _make

# List of valid names for the first argument in a class method.
valid-classmethod-first-arg=cls

# List of valid names for the first argument in a metaclass class method.
valid-metaclass-classmethod-first-arg=cls


[IMPORTS]

# List of modules that can be imported at any level, not just the top level
# one.
allow-any-import-level=

# Allow wildcard imports from modules that define __all__.
allow-wildcard-with-all=no

# Analyse import fallback blocks. This can be used to support both Python 2 and
# 3 compatible code, which means that the block might have code that exists
# only in one or another interpreter, leading to false positives when analysed.
analyse-fallback-blocks=no

# Deprecated modules which should not be used, separated by a comma.
deprecated-modules=optparse,tkinter.tix

# Create a graph of external dependencies in the given file (report RP0402 must
# not be disabled).
ext-import-graph=

# Create a graph of every (i.e. internal and external) dependencies in the
# given file (report RP0402 must not be disabled).
import-graph=

# Create a graph of internal dependencies in the given file (report RP0402 must
# not be disabled).
int-import-graph=

# Force import order to recognize a module as part of the standard
# compatibility libraries.
known-standard-library=

# Force import order to recognize a module as part of a third party library.
known-third-party=enchant

# Couples of modules and preferred modules, separated by a comma.
preferred-modules=


[EXCEPTIONS]

# Exceptions that will emit a warning when being caught. Defaults to
# "BaseException, Exception".
overgeneral-exceptions=BaseException,
                       Exception

%%sh
ls -ad ~/.*rc

/home/bobl/.asoundrc
/home/bobl/.bashrc
/home/bobl/.cvsrc
/home/bobl/.dmrc
/home/bobl/.fbrc
/home/bobl/.gqcamrc
/home/bobl/.hxplayerrc
/home/bobl/.idlerc
/home/bobl/.joverc
/home/bobl/.mcoprc
/home/bobl/.mtinkrc
/home/bobl/.mtoolsrc
/home/bobl/.muttrc
/home/bobl/.nvidia-settings-rc
/home/bobl/.realplayerrc
/home/bobl/.ripperXrc
/home/bobl/.sversionrc
/home/bobl/.xdvirc
/home/bobl/.xpdfrc

pdb

Here's a little program, "4_faro_py2k.py" written in Python 2K.

%%file demos/4_faro_py2k.py
"""
This program explores the non-randomness of the traditional 'faro'
method of card shuffling.  No matter how many cards there are in a
deck, there is always a finite number of faro shuffles that will
restore their original order.  (Of course, a faro shuffle is very hard
to do in practice!)
"""


def faroShuffle(oldDeck):
    """
    performs a 'faro' or 'perfect' shuffle

    `oldDeck` (a sequence) is the original deck.  The shuffled deck is
    returned.  It does an 'out' shuffle (top and bottom cards
    preserved).
    """
    # Look how easy Python makes this!
    n = len(oldDeck)
    newDeck = n*[ 0 ] # initializes result
    newDeck[0:n-1:2] = oldDeck[:n/2] # insert left hand
    newDeck[1:n:2]   = oldDeck[n/2:] # insert right hand
    return newDeck

def countShuffles(nCards):
    """
    counts the number of faro shuffles required to restore a deck

    `nCards` is the number of cards in the deck.  It must be an even
    positive int.
    """
    originalDeck = range(nCards)
    currentDeck = originalDeck
    shuffleCount = 0
    while True:
        shuffledDeck = faroShuffle(currentDeck)
        shuffleCount += 1
        if shuffledDeck == originalDeck:
            return shuffleCount
        currentDeck = shuffledDeck


nCards = 52
print '''
The ordering of a deck of {0} cards is restored after {1} faro shuffles.
'''.format(nCards, countShuffles(nCards))

Overwriting demos/4_faro_py2k.py

Run with Python2K, it runs fine:

%%sh
python2 demos/4_faro_py2k.py

The ordering of a deck of 52 cards is restored after 8 faro shuffles.

But in Python3K, there are problems. (If you're running this in your own IPython Notebook, session, you may need to interrupt the kernel.)

%%sh
python3 demos/4_faro_py2k.py || exit 0 # make Notebook ignore error status

  File "demos/4_faro_py2k.py", line 46
    print '''
The ordering of a deck of {0} cards is restored after {1} faro shuffles.
'''.format(nCards, countShuffles(nCards))
          ^
SyntaxError: invalid syntax

So one thing we clearly have to do is change the print statement to a function call (in 5_faro_new_print.py):

%%file demos/5_faro_new_print.py
"""
This program explores the non-randomness of the traditional 'faro'
method of card shuffling.  No matter how many cards there are in a
deck, there is always a finite number of faro shuffles that will
restore their original order.  (Of course, a faro shuffle is very hard
to do in practice!)
"""

def faroShuffle(oldDeck):
    """
    performs a 'faro' or 'perfect' shuffle

    `oldDeck` (a sequence) is the original deck.  The shuffled deck is
    returned.  It does an 'out' shuffle (top and bottom cards
    preserved).
    """
    # Look how easy Python makes this!
    n = len(oldDeck)
    newDeck = n*[ 0 ] # initializes result
    newDeck[0:n-1:2] = oldDeck[:n/2] # insert left hand
    newDeck[1:n:2]   = oldDeck[n/2:] # insert right hand
    return newDeck


def countShuffles(nCards):
    """
    counts the number of faro shuffles required to restore a deck

    `nCards` is the number of cards in the deck.  It must be an even
    positive int.
    """
    originalDeck = range(nCards)
    currentDeck = originalDeck
    shuffleCount = 0
    while True:
        shuffledDeck = faroShuffle(currentDeck)
        shuffleCount += 1
        if shuffledDeck == originalDeck:
            return shuffleCount
        currentDeck = shuffledDeck


if __name__ == '__main__':
    nCards = 52
    print('''
The ordering of a deck of {0} cards is restored after {1} faro shuffles.
'''[1:-1].format(nCards, countShuffles(nCards)))

Overwriting demos/5_faro_new_print.py

And if we run this...

%%sh
python3 demos/5_faro_new_print.py || exit 0 # make Notebook ignore error status

Traceback (most recent call last):
  File "demos/5_faro_new_print.py", line 47, in <module>
    '''[1:-1].format(nCards, countShuffles(nCards)))
  File "demos/5_faro_new_print.py", line 36, in countShuffles
    shuffledDeck = faroShuffle(currentDeck)
  File "demos/5_faro_new_print.py", line 20, in faroShuffle
    newDeck[0:n-1:2] = oldDeck[:n/2] # insert left hand
TypeError: slice indices must be integers or None or have an __index__ method

So what's the error here? We fix that and the result (in 6_faro_floor_div.py) is:

%%file demos/6_faro_floordiv.py
"""
This program explores the non-randomness of the traditional 'faro'
method of card shuffling.  No matter how many cards there are in a
deck, there is always a finite number of faro shuffles that will
restore their original order.  (Of course, a faro shuffle is very hard
to do in practice!)
"""

def faroShuffle(oldDeck):
    """
    performs a 'faro' or 'perfect' shuffle

    `oldDeck` (a sequence) is the original deck.  The shuffled deck is
    returned.  It does an 'out' shuffle (top and bottom cards
    preserved).
    """
    # Look how easy Python makes this!
    n = len(oldDeck)
    newDeck = n*[ 0 ] # initializes result
    newDeck[0:n-1:2] = oldDeck[:n//2] # insert left hand
    newDeck[1:n:2]   = oldDeck[n//2:] # insert right hand
    return newDeck


def countShuffles(nCards):
    """
    counts the number of faro shuffles required to restore a deck

    `nCards` is the number of cards in the deck.  It must be an even
    positive int.
    """
    originalDeck = range(nCards)
    currentDeck = originalDeck
    shuffleCount = 0
    while True:
        shuffledDeck = faroShuffle(currentDeck)
        shuffleCount += 1
        if shuffledDeck == originalDeck:
            return shuffleCount
        currentDeck = shuffledDeck


if __name__ == '__main__':
    nCards = 52
    print('''
The ordering of a deck of {0} cards is restored after {1} faro shuffles.
'''[1:-1].format(nCards, countShuffles(nCards)))

Overwriting demos/6_faro_floordiv.py

But if we try to run this, the code starts running but seems to be looping indefinitely. (If you're running this in your own IPython Notebook, session, be prepared to interrupt the kernel.)

This is a job for pdb. By importing that module, we can, for instance, start the debugger at any time during execution.

%%file demos/7_faro_for_pdb.py
"""
This program explores the non-randomness of the traditional 'faro'
method of card shuffling.  No matter how many cards there are in a
deck, there is always a finite number of faro shuffles that will
restore their original order.  (Of course, a faro shuffle is very hard
to do in practice!)
"""
import pdb

def faroShuffle(oldDeck):
    """
    performs a 'faro' or 'perfect' shuffle

    `oldDeck` (a sequence) is the original deck.  The shuffled deck is
    returned.  It does an 'out' shuffle (top and bottom cards
    preserved).
    """
    # Look how easy Python makes this!
    n = len(oldDeck)
    newDeck = n*[ 0 ] # initializes result
    newDeck[0:n-1:2] = oldDeck[:n//2] # insert left hand
    newDeck[1:n:2]   = oldDeck[n//2:] # insert right hand
    return newDeck


def countShuffles(nCards):
    """
    counts the number of faro shuffles required to restore a deck

    `nCards` is the number of cards in the deck.  It must be an even
    positive int.
    """
    originalDeck = range(nCards)
    currentDeck = originalDeck
    shuffleCount = 0
    while True:
        shuffledDeck = faroShuffle(currentDeck)
        shuffleCount += 1
        if shuffleCount == 8:
            pdb.set_trace()
        if shuffledDeck == originalDeck:
            return shuffleCount
        currentDeck = shuffledDeck


if __name__ == '__main__':
    nCards = 52
    print('''
The ordering of a deck of {0} cards is restored after {1} faro shuffles.
'''[1:-1].format(nCards, countShuffles(nCards)))

And here's the final version:

%%file demos/8_faro_fixed.py
"""
This program explores the non-randomness of the traditional 'faro'
method of card shuffling.  No matter how many cards there are in a
deck, there is always a finite number of faro shuffles that will
restore their original order.  (Of course, a faro shuffle is very hard
to do in practice!)
"""

from print_whence import printWhence


def faroShuffle(oldDeck, outShuffle=True):
    """
    performs a 'faro' or 'perfect' shuffle

    `oldDeck` (a sequence) is the original deck.  The shuffled deck is
    returned.  Iff `outShuffle` is True, it does an 'out' shuffle (top
    and bottom cards preserved).  If it is False, it does an 'in'
    shuffle.
    """
    # Look how easy Python makes this!
    n = len(oldDeck)
    newDeck = n*[ 0 ] # initializes result
    if outShuffle:
        newDeck[0:n-1:2] = oldDeck[:n//2] # insert left hand
        newDeck[1:n:2]   = oldDeck[n//2:] # insert right hand
    else:
        newDeck[0:n-1:2] = oldDeck[n//2:] # insert left hand
        newDeck[1:n:2]   = oldDeck[:n//2] # insert right hand
    return newDeck


def countShuffles(nCards):
    """
    counts the number of faro shuffles required to restore a deck

    `nCards` is the number of cards in the deck.  It must be an even
    positive int.
    """
    originalDeck = list(range(nCards))
    currentDeck = originalDeck
    shuffleCount = 0
    while True:
        shuffledDeck = faroShuffle(currentDeck)
        shuffleCount += 1
        if shuffledDeck == originalDeck:
            return shuffleCount
        currentDeck = shuffledDeck


if __name__ == '__main__':
    nCards = 52
    print('The ordering of a deck of {} cards is'
           ' restored after {} faro shuffles.'.format(
               nCards, countShuffles(nCards)))

Overwriting demos/8_faro_fixed.py

%%sh
python3 demos/8_faro_fixed.py

The ordering of a deck of 52 cards is restored after 8 faro shuffles.

Scaffolding

Interactive debugging with pdb or another IDE is all well and good, but these days I use an alternative: scaffolding. Let's start with the module print_whence.py. (It's in the unit directory with the demos.) Here it is:

import os
import inspect


def printWhence(comment=""):
    """prints a tag identifying the caller's file and line number

    This is mainly for debugging.
    """
    currentFrame = inspect.currentframe()
    callerFrame = currentFrame.f_back
    traceback = inspect.getframeinfo(callerFrame)

    # We assume that all file names being debugged are unique, so we
    # strip off directory information (if any).
    (_, fname) = os.path.split(traceback.filename)

    print("==== at {}:{}".format(
            fname, traceback.lineno), end="")
    if comment:
        print(" ({}) ".format(comment), end="")
    print(" ====")

When called, it prints out the name and line number of the file it's called from. That's not very useful from Jupyter Notebook:

printWhence()

==== at <ipython-input-25-e4a2c44f6fde>:2 ====

But it is useful in demos (which we'll now illustrate).

I recommend its use in "scaffolding": Adding additional temporary code to a Python module to display its progress. I recommend the following pattern:

if 1 [ and condition(s) ]: printWhence([commentString]) [ print(*tagString*, *object*) ]+ if 1: quit() print()