Blog posts for tags/python

  1. Python and sendfile

    sendfile(2) is a UNIX system call which provides a "zero-copy" way of copying data from one file descriptor (a file) to another (a socket). Because this copying is done entirely within the kernel, sendfile(2) is more efficient than the combination of and socket.send(), which requires transferring data to and from user space. This copying of the data twice imposes some performance and resource penalties which sendfile(2) syscall avoids; it also results in a single system call (and thus only one context switch), rather than the series of read(2) / write(2) system calls (each system call requiring a context switch) used internally for the data copying. A more exhaustive explanation of how sendfile(2) works is available here, but long story short is that sending a file with sendfile() is usually twice as fast than using plain socket.send(). Typical applications which can benefit from using sendfile() are FTP and HTTP servers.


    I recently contributed a patch for Python's socket module which adds a high-level socket.sendfile() method (see full discussion at BPO-17552). socket.sendfile() will transmit a file until EOF is reached by attempting to use os.sendfile(), if available, else it falls back on using plain socket.send(). Internally, it takes care of handling socket timeouts and provides two optional parameters to move the file offset or to send only a limited amount of bytes. I came up with this idea because getting all of that right is a bit tricky, so a generic wrapper seemed to be convenient to have. socket.sendfile() will make its appearance in Python 3.5.

    sendfile and Python

    sendfile(2) made its first appearance into the Python stdlib kind of late: Python 3.3. It was contributed by Ross Lagerwall and me in BPO-10882. Since the patch didn't make it into python 2.X and I wanted to use sendfile() in pyftpdlib I later decided to release it as a stand alone module working with older (2.5+) Python versions (see pysendfile project). Starting with version 3.5, Python will hopefully start using sendfile() more extensively, in details:

    Also, Windows provides something similar to sendfile(2): TransmitFile. Now that socket.sendfile() is in place it seems natural to add support for it as well (see BPO-21721).

    Backport to Python 2.6 and 2.7

    For those of you who are interested in using socket.sendfile() with older Python 2.6 and 2.7 versions here's a backport. It requires pysendfile module to be installed. Full code including tests is hosted here.

    #!/usr/bin/env python
    This is a backport of socket.sendfile() for Python 2.6 and 2.7.
    socket.sendfile() will be included in Python 3.5:
    >>> import socket
    >>> file = open("somefile.bin", "rb")
    >>> sock = socket.create_connection(("localhost", 8021))
    >>> sendfile(sock, file)
    import errno
    import io
    import os
    import select
    import socket
        memoryview  # py 2.7 only
    except NameError:
        memoryview = lambda x: x
    if == 'posix':
        import sendfile as pysendfile  # requires "pip install pysendfile"
        pysendfile = None
    _RETRY = frozenset((errno.EAGAIN, errno.EALREADY, errno.EWOULDBLOCK,
    class _GiveupOnSendfile(Exception):
    if pysendfile is not None:
        def _sendfile_use_sendfile(sock, file, offset=0, count=None):
            _check_sendfile_params(sock, file, offset, count)
            sockno = sock.fileno()
                fileno = file.fileno()
            except (AttributeError, io.UnsupportedOperation) as err:
                raise _GiveupOnSendfile(err)  # not a regular file
                fsize = os.fstat(fileno).st_size
            except OSError:
                raise _GiveupOnSendfile(err)  # not a regular file
            if not fsize:
                return 0  # empty file
            blocksize = fsize if not count else count
            timeout = sock.gettimeout()
            if timeout == 0:
                raise ValueError("non-blocking sockets are not supported")
            # poll/select have the advantage of not requiring any
            # extra file descriptor, contrarily to epoll/kqueue
            # (also, they require a single syscall).
            if hasattr(select, 'poll'):
                if timeout is not None:
                    timeout *= 1000
                pollster = select.poll()
                pollster.register(sockno, select.POLLOUT)
                def wait_for_fd():
                    if pollster.poll(timeout) == []:
                        raise socket._socket.timeout('timed out')
                # call select() once in order to solicit ValueError in
                # case we run out of fds
          [], [sockno], [], 0)
                except ValueError:
                    raise _GiveupOnSendfile(err)
                def wait_for_fd():
                    fds =[], [sockno], [], timeout)
                    if fds == ([], [], []):
                        raise socket._socket.timeout('timed out')
            total_sent = 0
            # localize variable access to minimize overhead
            os_sendfile = pysendfile.sendfile
                while True:
                    if timeout:
                    if count:
                        blocksize = count - total_sent
                        if blocksize <= 0:
                        sent = os_sendfile(sockno, fileno, offset, blocksize)
                    except OSError as err:
                        if err.errno in _RETRY:
                            # Block until the socket is ready to send some
                            # data; avoids hogging CPU resources.
                            if total_sent == 0:
                                # We can get here for different reasons, the main
                                # one being 'file' is not a regular mmap(2)-like
                                # file, in which case we'll fall back on using
                                # plain send().
                                raise _GiveupOnSendfile(err)
                            raise err
                        if sent == 0:
                            break  # EOF
                        offset += sent
                        total_sent += sent
                return total_sent
                if total_sent > 0 and hasattr(file, 'seek'):
        def _sendfile_use_sendfile(sock, file, offset=0, count=None):
            raise _GiveupOnSendfile(
                "sendfile() not available on this platform")
    def _sendfile_use_send(sock, file, offset=0, count=None):
        _check_sendfile_params(sock, file, offset, count)
        if sock.gettimeout() == 0:
            raise ValueError("non-blocking sockets are not supported")
        if offset:
        blocksize = min(count, 8192) if count else 8192
        total_sent = 0
        # localize variable access to minimize overhead
        file_read =
        sock_send = sock.send
            while True:
                if count:
                    blocksize = min(count - total_sent, blocksize)
                    if blocksize <= 0:
                data = memoryview(file_read(blocksize))
                if not data:
                    break  # EOF
                while True:
                        sent = sock_send(data)
                    except OSError as err:
                        if err.errno in _RETRY:
                        total_sent += sent
                        if sent < len(data):
                            data = data[sent:]
            return total_sent
            if total_sent > 0 and hasattr(file, 'seek'):
       + total_sent)
    def _check_sendfile_params(sock, file, offset, count):
        if 'b' not in getattr(file, 'mode', 'b'):
            raise ValueError("file should be opened in binary mode")
        if not sock.type & socket.SOCK_STREAM:
            raise ValueError("only SOCK_STREAM type sockets are supported")
        if count is not None:
            if not isinstance(count, int):
                raise TypeError(
                    "count must be a positive integer (got %s)" % repr(count))
            if count <= 0:
                raise ValueError(
                    "count must be a positive integer (got %s)" % repr(count))
    def sendfile(sock, file, offset=0, count=None):
        """sendfile(sock, file[, offset[, count]]) -> sent
        Send a *file* over a connected socket *sock* until EOF is
        reached by using high-performance sendfile(2) and return the
        total number of bytes which were sent.
        *file* must be a regular file object opened in binary mode.
        If sendfile() is not available (e.g. Windows) or file is
        not a regular file socket.send() will be used instead.
        *offset* tells from where to start reading the file.
        If specified, *count* is the total number of bytes to transmit
        as opposed to sending the file until EOF is reached.
        File position is updated on return or also in case of error in
        which case file.tell() can be used to figure out the number of
        bytes which were sent.
        The socket must be of SOCK_STREAM type.
        Non-blocking sockets are not supported.
            return _sendfile_use_sendfile(sock, file, offset, count)
        except _GiveupOnSendfile:
            return _sendfile_use_send(sock, file, offset, count)
  2. psutil 2.0

    The time has finally come: psutil 2.0 is out! This is a release which took me a considerable amount of effort and careful thinking during the past 4 months as I went through a major rewrite and reorganization of both python and C extension modules. To get a sense of how much has changed you can compare the differences with old 1.2.1 version by running "hg diff -r release-1.2.1:release-2.0.0" which will produce more than 22,000 lines of output! In those 22k lines I tried to nail down all the quirks the project had accumulated since its start 4 years ago and the resulting code base is now cleaner than ever, more manageable and fully compliant with PEP-7 and PEP-8 guidelines. There were some difficult decisions because many of the changes I introduced are not backward compatible so I was concerned with the pain this may cause existing users. I kind of still am, but I'm sure the transition will be well perceived on the long run as it will result in more manageable user code. OK, enough with the preface and let's see what changed.

    API changes

    I already wrote a detailed blog post about what changed so I recommend you to use that as the official reference on how to port your code.

    RST documentation

    I've never been happy with old doc hosted on Google Code. The markup language provided by Google is pretty limited, plus it's not put under revision control. New doc is more detailed, it uses reStructuredText as the markup language, it lives in the same code repository as psutil's and it is hosted on the excellent readthedocs web site:

    Physical CPUs count

    You're now able to distinguish between logical and physical CPUs:

    >>> psutil.cpu_count()  # logical
    >>> psutil.cpu_count(logical=False)  # physical cores only

    Full story is in issue 427.

    Process instances are hashable

    Basically this means process instances can now be checked for equality and can be used with set()s:

    >>> p1 = psutil.Process()
    >>> p2 = psutil.Process()
    >>> p1 == p2
    >>> set((p1, p2))
    set([<psutil.Process(pid=8217, name='python') at 140007043550608>])

    Full story is in issue 452.


    • #477: process cpu_percent() is about 30% faster.
    • #478: (Linux) almost all APIs are about 30% faster on Python 3.X.

    Other improvements and bugfixes

    • #424: Windows installers for Python 3.X 64-bit
    • #447: psutil.wait_procs() timeout parameter is now optional.
    • #459: a Makefile is now available for running tests and other repetitive tasks (also on Windows)
    • #463: timeout parameter of cpu_percent* functions default to 0.0 because it was a common trap to introduce slowdowns.
    • #340: (Windows) process open_files() no longer hangs.
    • #448: (Windows) fixed a memory leak affecting children() and ppid() methods.
    • #461: namedtuples are now pickle-able.
    • #474: (Windows) Process.cpu_percent() is no longer capped at 100%

    OK, that's all folks. I hope you will enjoy this new version and report your feedback.

  3. Reimplementing netstat in Python

    psutil 2.1.0 is out and with it I finally managed to implement something I've been wanting to have for a long time: netstat-like functionalities (see ticket). Similarly to "netstat -antp" on UNIX you can now list system-wide connections in pure python and also determine what process (PID) is using a particular connection:

    >>> import psutil
    >>> from pprint import pprint as pp
    >>> pp(psutil.net_connections())
    [sconn(fd=-1, family=2, type=1, laddr=('', 587), raddr=(), status='LISTEN', pid=None),
     sconn(fd=-1, family=2, type=1, laddr=('', 6379), raddr=(), status='LISTEN', pid=None),
     sconn(fd=-1, family=2, type=1, laddr=('', 53), raddr=(), status='LISTEN', pid=None),
     sconn(fd=-1, family=2, type=1, laddr=('', 53), raddr=(), status='LISTEN', pid=None),
     sconn(fd=-1, family=2, type=1, laddr=('', 631), raddr=(), status='LISTEN', pid=None),
     sconn(fd=-1, family=2, type=1, laddr=('', 25), raddr=(), status='LISTEN', pid=None),
     sconn(fd=-1, family=2, type=1, laddr=('', 3389), raddr=(), status='LISTEN', pid=None),
     sconn(fd=17, family=2, type=1, laddr=('', 34785), raddr=(), status='LISTEN', pid=3591),
     sconn(fd=15, family=2, type=1, laddr=('', 56359), raddr=(), status='LISTEN', pid=3591),
     sconn(fd=-1, family=10, type=2, laddr=('::', 56720), raddr=(), status='NONE', pid=None)]

    This is yet another functionality which can be used for monitoring purposes. For example, say you want to make sure your HTTP server is running on port 80, you can do something like this:

    import psutil
    def check_listening_port(port):
        """Return True if the given TCP port is busy and in LISTEN mode."""
        for conn in psutil.net_connections(kind='tcp'):
            if conn.laddr[1] == port and conn.status == psutil.CONN_LISTEN:
                return True
        return False

    Netstat in pure python

    Here it is, in 65 lines of code: Pretty neat right? ;-)


    As always, each platform required its own, different, implementation. Luckily for some platforms (OSX, Windows) I was able to reuse and customize some code from the existing Process.connections() implementation which was already in place. For those of you who are interested in knowing how this was done here's the source code references:

    Hopefully this will help whoever needs to do this into another language. The only platform where this is sort of clunky is OSX, which does not expose anything to list all system-wide sockets in a single shot, so you're forced to query each process. That means you'll need root privileges otherwise you'll get an access denied error. For what it's worth, I took a look at lsof and it has the same limitation and netstat runs with SUID. Well, I guess this is it. I'll leave you with some docs. For the next one I'm planning on working on a couple of other network-related functionalities: "ifconfig" and NIC speeds. But that's for another time...

  4. psutil 2.0 API redesign

    This my second blog post is going to be about psutil 2.0, a major release in which I decided to reorganize the existing API for the sake of consistency. At the time of writing psutil 2.0 is still under development and the intent of this blog post is to serve as an official reference which describes how you should port your existent code base. In doing so I will also explain why I decided to make these changes. Despite many APIs will still be available as aliases pointing to the newer ones, the overall changes are numerous and many of them are not backward compatible. I'm sure many people will be sorely bitten but I think this is for the better and it needed to be done, hopefully for the first and last time. OK, here we go now.

    Module constants turned into functions

    What changed

    Old name Replacement
    psutil.BOOT_TIME psutil.boot_time()
    psutil.NUM_CPUS psutil.cpu_count()
    psutil.TOTAL_PHYMEM psutil.virtual_memory().total

    Why I did it

    I already talked about this more extensively in this blog post. In short: other than introducing unnecessary slowdowns, calculating a module level constant at import time is dangerous because in case of error the whole app will crash. Also, the represented values may be subject to change (think about the system clock) but the constant cannot be updated. Thanks to this hack accessing the old constants still works and produces a DeprecationWarning.

    Module functions renamings

    What changed

    Old name Replacement
    psutil.get_boot_time() psutil.boot_time()
    psutil.get_pid_list() psutil.pids()
    psutil.get_users() psutil.users()

    Why I did it

    They were the only module level functions having a get_ prefix. All others do not.

    Process class' methods renamings

    All methods lost their get_ and set_ prefixes. A single method can now be used for both getting and setting (if a value is passed). Assuming p = psutil.Process():

    Old name Replacement
    p.get_children() p.children()
    p.get_connections() p.connections()
    p.get_cpu_affinity() p.cpu_affinity()
    p.get_cpu_percent() p.cpu_percent()
    p.get_cpu_times() p.cpu_times()
    p.get_io_counters() p.io_counters()
    p.get_ionice() p.ionice()
    p.get_memory_info() p.memory_info()
    p.get_ext_memory_info() p.memory_info_ex()
    p.get_memory_maps() p.memory_maps()
    p.get_memory_percent() p.memory_percent()
    p.get_nice() p.nice()
    p.get_num_ctx_switches() p.num_ctx_switches()
    p.get_num_fds() p.num_fds()
    p.get_num_threads() p.num_threads()
    p.get_open_files() p.open_files()
    p.get_rlimit() p.rlimit()
    p.get_threads() p.threads()
    p.getcwd() p.cwd() for set_* methods:

    Old name Replacement
    p.set_cpu_affinity() p.cpu_affinity(cpus)
    p.set_ionice() p.ionice(ioclass, value=None)
    p.set_nice() p.nice(value)
    p.set_rlimit() p.rlimit(resource, limits=None)

    Why I did it

    I wanted to be consistent with system-wide module level functions which have no get_ prefix. After I got rid of get_ prefixes removing also set_ seemed natural and helped diminish the number of methods.

    Process properties are now methods

    What changed

    Assuming p = psutil.Process():

    Old name Replacement
    p.cmdline p.cmdline()
    p.create_time p.create_time()
    p.exe p.exe()
    p.gids p.gids()
    p.parent p.parent()
    p.ppid p.ppid()
    p.status p.status()
    p.uids p.uids()
    p.username p.username()

    Why I did it

    Different reasons:

    • Having a mixed API which uses both properties and methods for no particular reason is confusing and messy as you don't know whether to use "()" or not (see here).
    • It is usually expected from a property to not perform many computations internally whereas psutil actually invokes a function every time it is accessed. This has two drawbacks: * you may get an exception just by accessing the property (e.g. "" may raise NoSuchProcess or AccessDenied) * you may erroneously think properties are cached but this is true only for name, exe, and create_time.

    CPU percent intervals

    What changed

    The timeout parameter of cpu_percent* functions now defaults to 0.0 instead of 0.1. The functions affected are:

    • psutil.Process.cpu_percent()
    • psutil.cpu_percent()
    • psutil.cpu_times_percent()

    Why I did it

    I originally set 0.1 as the default timeout because in order to get a meaningful percent value you need to wait some time. Having an API which "sleeps" by default is risky though, because it's easy to forget it does so. That is particularly problematic when calling cpu_percent() for all processes: it's very easy to forget about specifying timeout=0 resulting in dramatic slowdowns which are hard to spot. For example, this code snippet might take different seconds to complete depending on the number of active processes:

    >>> # this will be slow
    >>> for p in psutil.process_iter():
    ...    print(p.cpu_percent())

    Migration strategy

    Except for Process properties (name, exe, cmdline, etc.) all the old APIs are still available as aliases pointing to the newer names and raising DeprecationWarning. psutil will be very clear on what you should use instead of the deprecated API as long you start the interpreter with the "-Wd" option. This will enable deprecation warnings which were silenced in Python 2.7 (IMHO, from a developer standpoint this was a bad decision).

    giampaolo@ubuntu:/tmp$ python -Wd
    Python 2.7.3 (default, Sep 26 2013, 20:03:06)
    [GCC 4.6.3] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import psutil
    >>> psutil.get_pid_list()
    __main__:1: DeprecationWarning: psutil.get_pid_list is deprecated; use psutil.pids() instead
    [1, 2, 3, 6, 7, 13, ...]
    >>> p = psutil.Process()
    >>> p.get_cpu_times()
    __main__:1: DeprecationWarning: get_cpu_times() is deprecated; use cpu_times() instead
    pcputimes(user=0.08, system=0.03)

    If you have a solid test suite you can run tests and fix the warnings one by one. As for the the Process properties which were turned into methods it's more difficult because whereas psutil 1.2.1 returns the actual value, psutil 2.0.0 will return the bound method:

    # psutil 1.2.1
    >>> psutil.Process().name
    # psutil 2.0.0
    >>> psutil.Process().name
    <bound method of psutil.Process(pid=19816, name='python') at 139845631328144>

    What I would recommend if you want to drop support with 1.2.1 is to grep for ".name", ".exe" etc. and just replace them with ".exe()" and ".name()" one by one. If on the other hand you want to write a code which works with both versions I see two possibilities:

    • #1 check version info, like this:
    >>> PSUTIL2 = psutil.version_info >= (2, 0)
    >>> p = psutil.Process()
    >>> name = if PSUTIL2 else
    >>> exe = p.exe() if PSUTIL2 else p.exe
    • #2 get rid of all ".name", ".exe" occurrences you have in your code and use as_dict() instead:
    >>> p = psutil.Process()
    >>> pinfo = p.as_dict(attrs=["name", "exe"])
    >>> pinfo
    {'exe': '/usr/bin/python2.7', 'name': 'python'}
    >>> name = pinfo['name']
    >>> exe = pinfo['exe']

    New features introduced in 2.0.0

    Ok, enough with the bad news. =) psutil 2.0.0 is not only about code breakage. I also had the chance to integrate a bunch of interesting features.

    • #427: you're now able to distinguish between the number of logical and physical CPUs:
    >>> psutil.cpu_count()  # logical
    >>> psutil.cpu_count(logical=False)  # physical cores only
    • #452: process classes are now hashable and can be checked for equality. That means you can use Process objects with sets (finally!).
    • #447: psutil.wait_procs() "timeout" parameter is now optional
    • #461: functions returning namedtuples are now pickle-able
    • #459: a Makefile is now available to automatize repetitive tasks such as build, install, running tests etc. There's also a make.bat for Windows.
    • introduced unittest2 module as a requirement for running tests
  5. Making constants part of your API is evil

    One of the initial features which were included in psutil since day one (5 years ago) were system's boot time, number of CPUs and total physical memory. These metrics have one thing in common: they are (apparently) not supposed to change over time. That is why we (me and Jay) decided that exposing them as module constants calculated at import time was the way to go.

    >>> import psutil
    >>> psutil.NUM_CPUS
    >>> psutil.BOOT_TIME  # as seconds since the epoch
    >>> psutil.TOTAL_PHYMEM

    5 years later I regret that decision and I'm going to explain you why you don't want to do the same mistake.

    A constant should not change

    When we think of 'constants', our expectations are that they should not change over time. It may be obvious, but before thinking about introducing a constant be absolutely sure the value it represents is going to remain the same. Now, back then we thought these 3 metrics were not supposed to change, at least until the system is rebooted. Well, we were wrong: it turns out 2 of them actually can. Apparently virtualized systems can change physical installed memory at runtime (see here and here) and system boot time can easily be altered every time you update the system clock. In both of these cases, of course, the constants will not reflect the updated values.

    Doing things at import time is dangerous

    That's because import time usually means startup time and if something goes wrong the whole application will crash. In general the only reason for a module to crash at import time is because of a missing dependancy or because it's not supposed to run on that platform in the first place. Now, here's a couple of bug reports which were collected over time: issue 188, issue 313. The inconvenience was so severe that I had to release different fixed versions ASAP and the fix consisted of a stinky workaround. That's when I started thinking about getting rid of those constants once and for all and introduce functions instead. But how to do that without breaking everybody's code?

    Backward compatibility matters

    Now here's the crucial part: every time you deliver a library to someone else you just cannot remove an API all of the sudden, especially if they are 3 and have been around since day one. It should first be deprecated, possibly turned into an alias pointing to a newer API and finally be removed after 1 or 2 major releases. Also, you want the deprecated API to explicitly raise a DeprecationWarning informing the user he's relying on something which will eventually be removed. With a module constant you cannot do any of that. What you would need is a module property.

    Module properties

    One of the greatest things about Python is that it's so dynamic that it lets you do all sort of nasty things with objects, including injecting names into modules (which are also objects) and make them behave like actual class properties! For this I have to thank Josiah Carlson who came up with this very simple yet effective solution:

    class ModuleWrapper(object):
        def __repr__(self):
            return repr(self._module)
        __str__ = __repr__
        def NUM_CPUS(self):
            msg = "NUM_CPUS constant is deprecated; use cpu_count() instead"
            warnings.warn(msg, category=DeprecationWarning, stacklevel=2)
            return cpu_count()
        def BOOT_TIME(self):
            msg = "BOOT_TIME constant is deprecated; " \
                  "use get_boot_time() instead"
            warnings.warn(msg, category=DeprecationWarning, stacklevel=2)
            return get_boot_time()
        def TOTAL_PHYMEM(self):
            msg = "TOTAL_PHYMEM constant is deprecated; " \
                  "use virtual_memory().total instead"
            warnings.warn(msg, category=DeprecationWarning, stacklevel=2)
            return virtual_memory().total
    mod = ModuleWrapper()
    mod.__dict__ = globals()
    mod._module = sys.modules[__name__]
    sys.modules[__name__] = mod

    You can put this at the bottom of your module and you'll have fully working module constants (tested on Python from 2.4 to 3.4)!

    EDIT: the only reason I applied this hack is to turn the old constants into aliases pointing to the newly introduced functions and produce a deprecation warning. That aside I can't think of a case where the use of a module property would be justified.

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