polyphrase

Why does my compiler complain "decimal constant is so large"?

2009-12-04T03:01:00.011+09:00

This article has been moved to http://www.polyphrase.com/2009/12/04/why-does-my-compiler-complain-decimal-constant-is-so-large.html.

Today my friend tweeted that "Why does my compiler complain about decimal constant is so large for -9223372036854775808?". To my shame, I misunderstood what he said at first. I replied to his tweet thus:

"It's overflow!"
"No. -9223372036854775808 is the minimum value of 64 bit integers."

He replied me back. he's right.

The number -9223372036854775808 (-2⁶³ -1) is the minimum value of 64 bit integers. So why the compiler emitted the warning? it's broken?

Definitely not.

In compiler's internal, the unadorned decimal constant "-9223372036854775808" is splitted into two tokens, an unary minus operator '-' and '9223372036854775808'.

The decimal number 9223372036854775808 is too large, and its type is interpreted deferently depending on which version of ISO C spec (C90 or C99?), or which data models (LP64, LLP64, ...?). So the compiler warned us that "warning: this decimal constant is unsigned only in ISO C90".

main.py to use Django 1.1 on Google AppEngine SDK 1.2.7

2009-12-01T01:24:00.003+09:00

I tried today to use Django 1.1 on Google AppEngine SDK 1.2.7. Here is the main.py for this purpose:

Thanks to these helpful articles:

AppleScript: Send link from Safari to Evernote.app

2008-12-15T22:02:00.004+09:00

I wrote a tiny AppleScript that helps to work with Mac OS X version of Evernote (Evernote.app). With this script, you can easily add link to web page opened in Safari.

How To Use

Launch Script Editor.app (at /Applications/AppleScript/Script Editor.app)
Just copy the script (below or here) from your browser, paste, and run.
Now you see a link to an opened web page is added in Evernote.

The Script

Some useful printf-style formats of java.util.Formatter

2008-10-15T10:49:00.002+09:00

java.util.Formatter which is introduced in J2SE 5.0 provides an interpreter for printf-style format strings. You can use pritf-style format through java.io.PrintStream instance.

int n = 1000000;
long millis = System.currentTimeMillis();
theLongRunningFunction(n);
System.out.printf("%d items - %d msecs elapsed\n",
    n, System.currentTimeMillis() - millis);

// => 1000000 items - 164 msecs elapsed

The elapsed seconds is formatted by using %f conversion and precision specifier.

int n = 1000000;
long millis = System.currentTimeMillis();
theLongRunningFunction(n);
System.out.printf("%d items - %.2f secs elapsed\n",
    n, (System.currentTimeMillis() - millis)/1000.0);

// => 1000000 items - 0.16 secs elapsed

The locale-specific grouping separators is also supported.

int n = 1000000;
long millis = System.currentTimeMillis();
theLongRunningFunction(n);
System.out.printf("%,d items - %.2f secs elapsed\n",
    n, (System.currentTimeMillis() - millis)/1000.0);

// => 1,000,000 items - 0.16 secs elapsed

Tokyo Street Cats

2008-09-27T19:15:00.003+09:00

Today, I went for a stroll through the streets of Kitano and Kitakarasuyama.

大きな地図で見る

Meeting lots of cats and having happy moments with them.

Hide and seek? :-)

Python version check

2008-09-26T21:33:00.001+09:00

This article has been moved to http://www.polyphrase.com/2008/09/26/python-version-check.html.

Just use sys.hexversion instead of the string equivalent (sys.version, sys.version_info, ...). It's flexible and fast.

>>> import sys
>>> sys.hexversion
33882608
>>> hex(sys.hexversion)
'0x20501f0'
>>> if sys.hexversion >= 0x2050100:
...   print "Python 2.5.1 or higher"
... 
Python 2.5.1 or higher

For example, the explicit relative imports is introduced in Python 2.5, you can check this function as:

HAS_RELATIVE_IMPORTS = (sys.hexversion >= 0x2050000)

NOTE: sys.hexversion is available since 1.5.2. It's not worth to support older versions.

pylint: disable/enable message for a block or statement

2008-09-24T19:34:00.003+09:00

This article has been moved to http://www.polyphrase.com/2008/09/24/how-to-disable-pylint-warnings-for-a-block-or-statement.html.

One of my favorite things about pylint is a feature that allows to disable/enable message for a block or statement.

For instance, consider the function below:

def scan_code(co, module):
    assert co and module
    if DEBUG: print co.co_filename
    ...

The function scan_code has so many debug prints. For performance reason, however, they are if DEBUG: print statements. Now pylint would report so many messages (C: means convention, for programming standard violation).

% pylint scan_code
...
C:107:scan_code: More than one statement on a single line
C:110:scan_code: More than one statement on a single line
C:114:scan_code: More than one statement on a single line
C:117:scan_code: More than one statement on a single line
C:123:scan_code: More than one statement on a single line

You can disable these messages for scan_code function. # pylint: disable-msg=... style comment can be used everywhere in the source code and affects a block or statement.


# pylint: disable-msg=C0321
def scan_code(co, module):
    assert co and module
    if DEBUG: print co.co_filename
    ...

The Power of Python's Generator expressions and Tuple pack/unpack

2008-09-23T21:44:00.003+09:00

This article has been moved to http://www.polyphrase.com/2008/09/23/the-power-of-pythons-generator-expressions-and-tuple-pack-unpack.html.

Python's Generator expressions and Tuple pack/unpack is more powerful than I thought.

for k, v in d.iteritems():
  if v > 123:
    yield k, v

is equivalent to:

return (item for item in d.iteritems() if item[1] > 123)

Some simple generators can be coded using Generator expressions, and Tuple pack/unpack is an easy way to return multiple values.

% python
Python 2.5.1 (r251:54863, Apr 15 2008, 22:57:26) 
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> d = {'apple': 256, 'hoge': 123, 'orange': 555}
>>> 
>>> def fn_yield():
...   for k, v in d.iteritems():
...     if v > 123:
...       yield k, v
... 
>>> def fn_generator():
...   return (item for item in d.iteritems() if item[1] > 123)
... 
>>> for k, v in fn_yield():
...   print k, v
... 
orange 555
apple 256
>>> for k, v in fn_generator():
...   print k, v
... 
orange 555
apple 256

Do you know Universal Newline Support in Python?

2008-09-22T14:29:00.004+09:00

This article has been moved to http://www.polyphrase.com/2008/09/22/do-you-know-universal-newline-support-in-python.html.

Do you know Universal Newline Support in Python? To be honest, I didn't know that. The PEP 278 describes in detail:

In a Python with universal newline support open() the mode parameter can also be "U", meaning "open for input as a text file with universal newline interpretation". Mode "rU" is also allowed, for symmetry with "rb". Mode "U" cannot be combined with other mode flags such as "+". Any line ending in the input file will be seen as a '\n' in Python, so little other code has to change to handle universal newlines.

Conversion of newlines happens in all calls that read data: read(), readline(), readlines(), etc.

Here is an example:

% printf 'Hello, World!\r\n' > /tmp/hello.txt
% cat /tmp/hello.txt 
Hello, World!
% python 
Python 2.5.1 (r251:54863, Apr 15 2008, 22:57:26) 
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> open('/tmp/hello.txt').read()
'Hello, World!\r\n'
>>> open('/tmp/hello.txt', 'U').read()
'Hello, World!\n'

If Python was built with the --with-universal-newlines option to configure (the default), File objects have newlines attribute, so testing whether the current Python has universal newline support can be done with:

if hasattr(sys.__stdout__, "newlines"):
    print "universal line endings"

How to compile Python source file with or without optimization

2008-09-21T12:39:00.004+09:00

py_compile and compileall module provides functions to generate a byte-code file from a source file. You can not, however, enable or disable byte code optimization at runtime.

The byte code optimization depends the built-in variable __debug__ in the current interpreter, and assignments to __debug__ are illegal. Python 2.2 (or later) is strict about that.

% python
Python 2.5.1 (r251:54863, Apr 15 2008, 22:57:26) 
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> __debug__
True
>>> __debug__ = False
  File "<stdin>", line 1
SyntaxError: can not assign to __debug__

So you can't enable/disable optimization flag without running another interpreter (using '-O' switch).

Example: Using `os.spawnv` to run and compile files with or without optimization

assertNone/assertNotNone

2008-09-15T10:39:00.003+09:00

Why Python unittest module lacks assertNone/assertNotNone? I think this lack would cause problems as API misuse. For example:

obj = factory.make_object()
self.assert_(obj) # must not be None (?)

I intended to say "factory.make_object must not return None", but It can fail even False and such an empty object (0, "", [], ...).

I should have written like:

obj = factory.make_object()
self.assert_(obj is not None) # must not be None!

Repeating that code, however, is so boring, a lot of the visual noise, and error-prone. Writing assertNone/assertNotNone:

class TestCase(unittest.TestCase):
    """Custom TestCase class"""

    def failUnlessNone(self, expr, msg=None):
        """Fail the test unless the expression is None."""
        if expr is not None: raise self.failureException, msg

    def failIfNone(self, expr, msg=None):
        """Fail the test if the expression is None."""
        if expr is None: raise self.failureException, msg

    # Synonyms for assertion methods
    assertNone = failUnlessNone
    assertNotNone = failIfNone

pylint: -r/--reports option doesn't work

2008-09-15T02:23:00.000+09:00

The following pylint command still outputs reports:

% pylint --reports=no module

versions and environment:

% uname -a
Darwin macbook3.local 9.4.0 Darwin Kernel Version 9.4.0: Mon Jun  9 19:30:53 PDT 2008; root:xnu-1228.5.20~1/RELEASE_I386 i386
% python --version
Python 2.5.1
% pylint --version
No config file found, using default configuration
pylint 0.15.0,
astng 0.17.3, common 0.35.0
Python 2.5.1 (r251:54863, Apr 15 2008, 22:57:26)
[GCC 4.0.1 (Apple Inc. build 5465)]

on Mac OS X 10.5.4.

I gave a quick look at the pylint source code. Removing optparse's default option fixes this problem (see below), but I don't know why.

--- pylint_orig/lint.py 2008-09-14 18:49:35.000000000 +0900
+++ /Library/Python/2.5/site-packages/pylint-0.15.0-py2.5.egg/pylint/lint.py 2008-09-14 18:50:07.000000000 +0900
@@ -184,7 +184,7 @@
 Reports (if any) will be written in a file name "pylint_global.[txt|html]".'}),
                
                ('reports',
-                {'default': 1, 'type' : 'yn', 'metavar' : '',
+                {'type' : 'yn', 'metavar' : '',
                  'short': 'r',
                  'group': 'Reports',
                  'help' : 'Tells wether to display a full report or only the\