python
an interpreted, interactive, object-oriented programming language
Synopsis
python [
-B ] [ -d ] [ -E ] [
-h ] [ -i ] [ -m
module-name ]
[ -O ] [ -OO ] [ -R ]
[ -Q argument ] [ -s ] [
-S ] [ -t ] [ -u ]
[ -v ] [ -V ] [ -W
argument ] [ -x ] [ -3 ] [
-? ]
[ -c command | script | - ]
[ arguments ]
add an example, a script, a trick and tips
examples
For Python2.x, `python -m SimpleHTTPServer` and Python3.x `python -m http.server`
##What does it do ?
Starts a webserver in the current directory that receives request on port 8000 by default
##Output:
Serving HTTP on 0.0.0.0 port 8000 ...
example added by Yaw
source
How to globally modify the default PYTHONPATH (sys.path)?
You might create a new file called
/etc/profile.d/local_python.sh
with the contents
PYTHONPATH="/usr/local/lib/python2.6/site-packages/":"${PYTHONPATH}"
export PYTHONPATH
Which will set the PYTHONPATH
variable for all
logged in users on your system.
source
Thoughts on best UNIX/Linux for python, R, C++
Gentoo!! Great package support, and highest performance possible
of any distro. Also support for strange hardware. Plus it's fun
to set up.
The C++ performance is almost exactly the same from Linux to OS
X. I haven't used R on either platform, but the fundamental
systems between OS X and linux are so similar that I wouldn't
think the platform would impact performance in any significant
way.
You can always download Linux and try it out. You can find a
distro that will work on your current Mac.
source
How can I run my python program directly from the shell?
You should probably rename your file main.py
to
internetScanner
. Extensions on *nix are purely
optional. It shouldn't matter here.
mv main.py internetScanner
Then, add the following line to this file, right at the
beginning:
#!/usr/bin/env python3
This will make sure that when the shell executes the file, it
will know to use python3
to interpret the content.
This is known as the Shebang.
Now, make the file executable:
chmod +x internetScanner
You can now run your program from within
/User/Desktop/project/internetScanner/
:
./internetScanner start
Your program will run in the foreground and continue running
until you press Ctrl-C. If you do not want
this, you can also start the program in the background, by adding
an ampersand after the command:
./internetScanner start &
This will let your program run, but you can continue to use your
shell. This is called job control, and there's a simple tutorial
about it here.
If you now want to be able to run the program from anywhere on
the system, you need to add the internetScanner
directory to your PATH
:
What are PATH and other environment variables, and how can I set
or use them?
source
Scripting: what is the easiest to extact a value in a tag of a XML file?
xml2 can convert xml to/from line-oriented format:
xml2 < pom.xml | grep /project/version= | sed 's/.*=//'
source
"bad interpreter: Permission denied" How can I prevent this error?
Long-shot, but make sure that /tmp
isn't mounted
with the noexec
mount option. That can give errors
like that.
source
determine where python is running from in linux
Use which python
.
$ which python
/usr/bin/python
C:\Users\anossovp>where python
c:\Python27\python.exe
source
When a python process is killed on OSX, why doesn't it kill the child processes?
On linux, when you kill a parent the child gets sent a sighup
which will generally kill it unless it was meant to stay alive as
a daemon in which case it will trap sighup. (I think this is why
one usually uses sighup to tell a daemon to refresh itself, since
it's conveneintly always trapped). On macosx I can't find
documentation but it appears that no sighup gets sent. As a
result the child process is orphaned and it's new parent is the
grandparent. The way you deal with this is you send the kill
signal to the process group of the parent not the parent process
itself. This will nuke all the children and grand children as
well with one caveate. If any child process does a setpgrp() or
setsid() then it escapes the process group membership. It will
not get the kill sent to it's old process group. Usually one need
not worry about the latter since it's usually intentional when
used to achieve that purpose.
source
About memory cache of Linux
I don't think you have an actual problem here.
Why do you care how much total memory is being used? Is it
hurting anything?
Linux tries to use your memory as efficiently as possible. If
you've got tons of free memory, and you're reading lots of files,
it'll use the free memory to cache those files. If you've got
tons of active memory, it'll use a lot less memory for caching.
That's the way it's supposed to work. If you don't want the
memory to be used at all, take the chips out of your computer.
There are some uncommon cases where lots of caching could be bad.
Maybe the burst of reading always happens right before a burst of
memory allocation. Maybe something about your specific case means
there's no possible benefit, and you need to squeeze out that
last 0.1% performance cost. Maybe you're on an embedded system
that uses RAM with a limited lifetime. If you have such a case,
tell us what it is.
source
How to diagnosis and resolve: /usr/lib64/libz.so.1: no version information available
From
What does the “no version information available” error from linux
dynamic linker mean?, pertaining to libpam :
The "no version information available" means that the library
version number is lower on the shared object. For example, if
your major.minor.patch number is 7.15.5 on the machine where
you build the binary, and the major.minor.patch number is
7.12.1 on the installation machine, ld will print the warning.
You can fix this by compiling with a library (headers and
shared objects) that matches the shared object version shipped
with your target OS. E.g., if you are going to install to
RedHat 3.4.6-9 you don't want to compile on Debian 4.1.1-21.
This is one of the reasons that most distributions ship for
specific linux distro numbers.
Otherwise, you can statically link. However, you don't want to
do this with something like PAM, so you want to actually
install a development environment that matches your client's
production environment (or at least install and link against
the correct library versions.)
source
How do *.pyc files execute?
Just because a file is marked executable does not mean that the
OS knows how to execute it. What happened here is that the .pyc
file inherited some of the permissions (or assumed default
filesystem permissions, for example, if you're on a FAT32 drive
in Linux).
In general, you don't execute .pyc files. Python stores them so
it doesn't have to recompile the code every time it is imported.
description
Python is an
interpreted, interactive, object-oriented programming
language that combines remarkable power with very clear
syntax. For an introduction to programming in Python you are
referred to the Python Tutorial. The Python Library
Reference documents built-in and standard types, constants,
functions and modules. Finally, the Python Reference Manual
describes the syntax and semantics of the core language in
(perhaps too) much detail. (These documents may be located
via the INTERNET RESOURCES below; they may be
installed on your system as well.)
Python’s
basic power can be extended with your own modules written in
C or C++. On most systems such modules may be dynamically
loaded. Python is also adaptable as an extension language
for existing applications. See the internal documentation
for hints.
Documentation
for installed Python modules and packages can be viewed by
running the pydoc program.
command line options
-B
Don’t write .py[co] files on import. See also
PYTHONDONTWRITEBYTECODE.
-c command
Specify the command to execute (see next section). This
terminates the option list (following options are passed as
arguments to the command).
-d
Turn on parser debugging output (for wizards only, depending on
compilation options).
-E
Ignore environment variables like PYTHONPATH and PYTHONHOME that
modify the behavior of the interpreter.
-h , -? , --help
Prints the usage for the interpreter executable and exits.
-i
When a script is passed as first argument or the -c option
is used, enter interactive mode after executing the script or the
command. It does not read the $PYTHONSTARTUP file. This can be
useful to inspect global variables or a stack trace when a script
raises an exception.
-m module-name
Searches sys.path for the named module and runs the
corresponding .py file as a script.
-O
Turn on basic optimizations. This changes the filename extension
for compiled (bytecode) files from .pyc to .pyo.
Given twice, causes docstrings to be discarded.
-OO
Discard docstrings in addition to the -O optimizations.
-R
Turn on "hash randomization", so that the hash() values of str,
bytes and datetime objects are "salted" with an unpredictable
pseudo-random value. Although they remain constant within an
individual Python process, they are not predictable between
repeated invocations of Python.
This is intended to provide protection against a denial of
service caused by carefully-chosen inputs that exploit the worst
case performance of a dict construction, O(n^2) complexity. See
http://www.ocert.org/advisories/ocert-2011-003.html for details.
-Q argument
Division control; see PEP 238. The argument must be one of "old"
(the default, int/int and long/long return an int or long), "new"
(new division semantics, i.e. int/int and long/long returns a
float), "warn" (old division semantics with a warning for int/int
and long/long), or "warnall" (old division semantics with a
warning for all use of the division operator). For a use of
"warnall", see the Tools/scripts/fixdiv.py script.
-s
Don’t add user site directory to sys.path.
-S
Disable the import of the module site and the
site-dependent manipulations of sys.path that it entails.
-t
Issue a warning when a source file mixes tabs and spaces for
indentation in a way that makes it depend on the worth of a tab
expressed in spaces. Issue an error when the option is given
twice.
-u
Force stdin, stdout and stderr to be totally unbuffered. On
systems where it matters, also put stdin, stdout and stderr in
binary mode. Note that there is internal buffering in
xreadlines(), readlines() and file-object iterators ("for line in
sys.stdin") which is not influenced by this option. To work
around this, you will want to use "sys.stdin.readline()" inside a
"while 1:" loop.
-v
Print a message each time a module is initialized, showing the
place (filename or built-in module) from which it is loaded. When
given twice, print a message for each file that is checked for
when searching for a module. Also provides information on module
cleanup at exit.
-V , --version
Prints the Python version number of the executable and exits.
-W argument
Warning control. Python sometimes prints warning message to
sys.stderr. A typical warning message has the following
form: file:line:
category: message. By default, each warning
is printed once for each source line where it occurs. This option
controls how often warnings are printed. Multiple -W
options may be given; when a warning matches more than one
option, the action for the last matching option is performed.
Invalid -W options are ignored (a warning message is
printed about invalid options when the first warning is issued).
Warnings can also be controlled from within a Python program
using the warnings module.
The simplest form of argument is one of the following
action strings (or a unique abbreviation): ignore
to ignore all warnings; default to explicitly request the
default behavior (printing each warning once per source line);
all to print a warning each time it occurs (this may
generate many messages if a warning is triggered repeatedly for
the same source line, such as inside a loop); module to
print each warning only the first time it occurs in each module;
once to print each warning only the first time it occurs
in the program; or error to raise an exception instead of
printing a warning message.
The full form of argument is
action:message:category:module:line.
Here, action is as explained above but only applies to
messages that match the remaining fields. Empty fields match all
values; trailing empty fields may be omitted. The message
field matches the start of the warning message printed; this
match is case-insensitive. The category field matches the
warning category. This must be a class name; the match test
whether the actual warning category of the message is a subclass
of the specified warning category. The full class name must be
given. The module field matches the (fully-qualified)
module name; this match is case-sensitive. The line field
matches the line number, where zero matches all line numbers and
is thus equivalent to an omitted line number.
-x
Skip the first line of the source. This is intended for a DOS
specific hack only. Warning: the line numbers in error messages
will be off by one!
-3
Warn about Python 3.x incompatibilities that 2to3 cannot
trivially fix.
environment variables
PYTHONHOME
Change the location of the standard Python libraries. By default,
the libraries are searched in ${prefix}/lib/python<version>
and ${exec_prefix}/lib/python<version>, where ${prefix} and
${exec_prefix} are installation-dependent directories, both
defaulting to /usr/local. When $PYTHONHOME is set to a
single directory, its value replaces both ${prefix} and
${exec_prefix}. To specify different values for these, set
$PYTHONHOME to ${prefix}:${exec_prefix}.
PYTHONPATH
Augments the default search path for module files. The format is
the same as the shell’s $PATH: one or more directory pathnames
separated by colons. Non-existent directories are silently
ignored. The default search path is installation dependent, but
generally begins with ${prefix}/lib/python<version> (see
PYTHONHOME above). The default search path is always appended to
$PYTHONPATH. If a script argument is given, the directory
containing the script is inserted in the path in front of
$PYTHONPATH. The search path can be manipulated from within a
Python program as the variable sys.path.
PYTHONSTARTUP
If this is the name of a readable file, the Python commands in
that file are executed before the first prompt is displayed in
interactive mode. The file is executed in the same name space
where interactive commands are executed so that objects defined
or imported in it can be used without qualification in the
interactive session. You can also change the prompts
sys.ps1 and sys.ps2 in this file.
PYTHONY2K
Set this to a non-empty string to cause the time module to
require dates specified as strings to include 4-digit years,
otherwise 2-digit years are converted based on rules described in
the time module documentation.
PYTHONOPTIMIZE
If this is set to a non-empty string it is equivalent to
specifying the -O option. If set to an integer, it is
equivalent to specifying -O multiple times.
PYTHONDEBUG
If this is set to a non-empty string it is equivalent to
specifying the -d option. If set to an integer, it is
equivalent to specifying -d multiple times.
PYTHONDONTWRITEBYTECODE
If this is set to a non-empty string it is equivalent to
specifying the -B option (don’t try to write
.py[co] files).
PYTHONINSPECT
If this is set to a non-empty string it is equivalent to
specifying the -i option.
PYTHONIOENCODING
If this is set before running the interpreter, it overrides the
encoding used for stdin/stdout/stderr, in the syntax
encodingname:errorhandler The
errorhandler part is optional and has the same meaning as
in str.encode. For stderr, the errorhandler
part is ignored; the handler will always be ´backslashreplace´.
PYTHONNOUSERSITE
If this is set to a non-empty string it is equivalent to
specifying the -s option (Don’t add the user site
directory to sys.path).
PYTHONUNBUFFERED
If this is set to a non-empty string it is equivalent to
specifying the -u option.
PYTHONVERBOSE
If this is set to a non-empty string it is equivalent to
specifying the -v option. If set to an integer, it is
equivalent to specifying -v multiple times.
PYTHONWARNINGS
If this is set to a comma-separated string it is equivalent to
specifying the -W option for each separate value.
PYTHONHASHSEED
If this variable is set to "random", the effect is the same as
specifying the -R option: a random value is used to seed
the hashes of str, bytes and datetime objects.
If PYTHONHASHSEED is set to an integer value, it is used as a
fixed seed for generating the hash() of the types covered by the
hash randomization. Its purpose is to allow repeatable hashing,
such as for selftests for the interpreter itself, or to allow a
cluster of python processes to share hash values.
The integer must be a decimal number in the range [0,4294967295].
Specifying the value 0 will lead to the same hash values as when
hash randomization is disabled.
files and directories
These are subject to difference depending on local installation
conventions; ${prefix} and ${exec_prefix} are
installation-dependent and should be interpreted as for GNU
software; they may be the same. On Debian GNU/{Hurd,Linux} the
default for both is /usr.
${exec_prefix}/bin/python
Recommended location of the interpreter.
${prefix}/lib/python<version>
${exec_prefix}/lib/python<version>
Recommended locations of the directories containing the standard
modules.
${prefix}/include/python<version>
${exec_prefix}/include/python<version>
Recommended locations of the directories containing the include
files needed for developing Python extensions and embedding the
interpreter.
~/.pythonrc.py
User-specific initialization file loaded by the user
module; not used by default or by most applications.
internet resources
Main website: http://www.python.org/
Documentation: http://docs.python.org/
Developer resources: http://docs.python.org/devguide/
Downloads: http://python.org/download/
Module repository: http://pypi.python.org/
Newsgroups: comp.lang.python, comp.lang.python.announce
interpreter interface
The interpreter interface resembles that of the UNIX shell: when
called with standard input connected to a tty device, it prompts
for commands and executes them until an EOF is read; when called
with a file name argument or with a file as standard input, it
reads and executes a script from that file; when called
with -c command, it executes the Python
statement(s) given as command. Here command may
contain multiple statements separated by newlines. Leading
whitespace is significant in Python statements! In
non-interactive mode, the entire input is parsed before it is
executed.
If available, the script name and additional arguments thereafter
are passed to the script in the Python variable sys.argv,
which is a list of strings (you must first import sys to
be able to access it). If no script name is given,
sys.argv[0] is an empty string; if -c is used,
sys.argv[0] contains the string ’-c’. Note that
options interpreted by the Python interpreter itself are not
placed in sys.argv.
In interactive mode, the primary prompt is ’>>>’; the
second prompt (which appears when a command is not complete) is
’...’. The prompts can be changed by assignment to sys.ps1
or sys.ps2. The interpreter quits when it reads an EOF at
a prompt. When an unhandled exception occurs, a stack trace is
printed and control returns to the primary prompt; in
non-interactive mode, the interpreter exits after printing the
stack trace. The interrupt signal raises the
KeyboardInterrupt exception; other UNIX signals are not
caught (except that SIGPIPE is sometimes ignored, in favor of the
IOError exception). Error messages are written to stderr.
licensing
Python is distributed under an Open Source license. See the file
"LICENSE" in the Python source distribution for information on
terms & conditions for accessing and otherwise using Python and
for a DISCLAIMER OF ALL WARRANTIES.
author
The Python
Software Foundation: http://www.python.org/psf