subprocess --- 子行程管理

原始碼：Lib/subprocess.py

The subprocess module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. This module intends to replace several older modules and functions:

os.system
os.spawn*

Information about how the subprocess module can be used to replace these modules and functions can be found in the following sections.

也參考

PEP 324 -- 提議 subprocess 模組的 PEP

可用性: not Android, not iOS, not WASI.

此模組在行動平台或 WebAssembly 平台上不支援。

使用 subprocess 模組

The recommended approach to invoking subprocesses is to use the run() function for all use cases it can handle. For more advanced use cases, the underlying Popen interface can be used directly.

subprocess.run(args, *, stdin=None, input=None, stdout=None, stderr=None, capture_output=False, shell=False, cwd=None, timeout=None, check=False, encoding=None, errors=None, text=None, env=None, universal_newlines=None, **other_popen_kwargs)

Run the command described by args. Wait for command to complete, then return a CompletedProcess instance.

The arguments shown above are merely the most common ones, described below in Frequently Used Arguments (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the Popen constructor - most of the arguments to this function are passed through to that interface. (timeout, input, check, and capture_output are not.)

If capture_output is true, stdout and stderr will be captured. When used, the internal Popen object is automatically created with stdout and stderr both set to PIPE. The stdout and stderr arguments may not be supplied at the same time as capture_output. If you wish to capture and combine both streams into one, set stdout to PIPE and stderr to STDOUT, instead of using capture_output.

A timeout may be specified in seconds, it is internally passed on to Popen.communicate(). If the timeout expires, the child process will be killed and waited for. The TimeoutExpired exception will be re-raised after the child process has terminated. The initial process creation itself cannot be interrupted on many platform APIs so you are not guaranteed to see a timeout exception until at least after however long process creation takes.

The input argument is passed to Popen.communicate() and thus to the subprocess's stdin. If used it must be a byte sequence, or a string if encoding or errors is specified or text is true. When used, the internal Popen object is automatically created with stdin set to PIPE, and the stdin argument may not be used as well.

If check is true, and the process exits with a non-zero exit code, a CalledProcessError exception will be raised. Attributes of that exception hold the arguments, the exit code, and stdout and stderr if they were captured.

If encoding or errors are specified, or text is true, file objects for stdin, stdout and stderr are opened in text mode using the specified encoding and errors or the io.TextIOWrapper default. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.

If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process' environment. It is passed directly to Popen. This mapping can be str to str on any platform or bytes to bytes on POSIX platforms much like os.environ or os.environb.

範例：

>>> subprocess.run(["ls", "-l"])  # 不捕捉輸出
CompletedProcess(args=['ls', '-l'], returncode=0)

>>> subprocess.run("exit 1", shell=True, check=True)
Traceback (most recent call last):
  ...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

>>> subprocess.run(["ls", "-l", "/dev/null"], capture_output=True)
CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n', stderr=b'')

在 3.5 版被加入.

在 3.6 版的變更: 新增 encoding 與 errors 參數。

在 3.7 版的變更: Added the text parameter, as a more understandable alias of universal_newlines. Added the capture_output parameter.

在 3.12 版的變更: Changed Windows shell search order for shell=True. The current directory and %PATH% are replaced with %COMSPEC% and %SystemRoot%\System32\cmd.exe. As a result, dropping a malicious program named cmd.exe into a current directory no longer works.

class subprocess.CompletedProcess

The return value from run(), representing a process that has finished.

args

The arguments used to launch the process. This may be a list or a string.

returncode

Exit status of the child process. Typically, an exit status of 0 indicates that it ran successfully.

A negative value -N indicates that the child was terminated by signal N (POSIX only).

stdout

Captured stdout from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stdout was not captured.

If you ran the process with stderr=subprocess.STDOUT, stdout and stderr will be combined in this attribute, and stderr will be None.

stderr

Captured stderr from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stderr was not captured.

check_returncode()

If returncode is non-zero, raise a CalledProcessError.

在 3.5 版被加入.

subprocess.DEVNULL

Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that the special file os.devnull will be used.

在 3.3 版被加入.

subprocess.PIPE

Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that a pipe to the standard stream should be opened. Most useful with Popen.communicate().

subprocess.STDOUT

Special value that can be used as the stderr argument to Popen and indicates that standard error should go into the same handle as standard output.

exception subprocess.SubprocessError

Base class for all other exceptions from this module.

在 3.3 版被加入.

exception subprocess.TimeoutExpired

Subclass of SubprocessError, raised when a timeout expires while waiting for a child process.

cmd

Command that was used to spawn the child process.

timeout

Timeout in seconds.

output

Output of the child process if it was captured by run() or check_output(). Otherwise, None. This is always bytes when any output was captured regardless of the text=True setting. It may remain None instead of b'' when no output was observed.

stdout

Alias for output, for symmetry with stderr.

stderr

Stderr output of the child process if it was captured by run(). Otherwise, None. This is always bytes when stderr output was captured regardless of the text=True setting. It may remain None instead of b'' when no stderr output was observed.

在 3.3 版被加入.

在 3.5 版的變更: stdout and stderr attributes added

exception subprocess.CalledProcessError

Subclass of SubprocessError, raised when a process run by check_call(), check_output(), or run() (with check=True) returns a non-zero exit status.

returncode

Exit status of the child process. If the process exited due to a signal, this will be the negative signal number.

cmd

Command that was used to spawn the child process.

output

Output of the child process if it was captured by run() or check_output(). Otherwise, None.

stdout

Alias for output, for symmetry with stderr.

stderr

Stderr output of the child process if it was captured by run(). Otherwise, None.

在 3.5 版的變更: stdout and stderr attributes added

Frequently Used Arguments

To support a wide variety of use cases, the Popen constructor (and the convenience functions) accept a large number of optional arguments. For most typical use cases, many of these arguments can be safely left at their default values. The arguments that are most commonly needed are:

args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell must be True (see below) or else the string must simply name the program to be executed without specifying any arguments.

stdin, stdout and stderr specify the executed program's standard input, standard output and standard error file handles, respectively. Valid values are None, PIPE, DEVNULL, an existing file descriptor (a positive integer), and an existing file object with a valid file descriptor. With the default settings of None, no redirection will occur. PIPE indicates that a new pipe to the child should be created. DEVNULL indicates that the special file os.devnull will be used. Additionally, stderr can be STDOUT, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.

If encoding or errors are specified, or text (also known as universal_newlines) is true, the file objects stdin, stdout and stderr will be opened in text mode using the encoding and errors specified in the call or the defaults for io.TextIOWrapper.

For stdin, line ending characters '\n' in the input will be converted to the default line separator os.linesep. For stdout and stderr, all line endings in the output will be converted to '\n'. For more information see the documentation of the io.TextIOWrapper class when the newline argument to its constructor is None.

If text mode is not used, stdin, stdout and stderr will be opened as binary streams. No encoding or line ending conversion is performed.

在 3.6 版的變更: 新增 encoding 與 errors 參數。

在 3.7 版的變更: 新增 text 參數作為 universal_newlines 的別名。

備註

The newlines attribute of the file objects Popen.stdin, Popen.stdout and Popen.stderr are not updated by the Popen.communicate() method.

If shell is True, the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of ~ to a user's home directory. However, note that Python itself offers implementations of many shell-like features (in particular, glob, fnmatch, os.walk(), os.path.expandvars(), os.path.expanduser(), and shutil).

在 3.3 版的變更: When universal_newlines is True, the class uses the encoding locale.getpreferredencoding(False) instead of locale.getpreferredencoding(). See the io.TextIOWrapper class for more information on this change.

備註

Read the Security Considerations section before using shell=True.

These options, along with all of the other options, are described in more detail in the Popen constructor documentation.

Popen Constructor

The underlying process creation and management in this module is handled by the Popen class. It offers a lot of flexibility so that developers are able to handle the less common cases not covered by the convenience functions.

class subprocess.Popen(args, bufsize=-1, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=True, shell=False, cwd=None, env=None, universal_newlines=None, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=(), *, group=None, extra_groups=None, user=None, umask=-1, encoding=None, errors=None, text=None, pipesize=-1, process_group=None)

Execute a child program in a new process. On POSIX, the class uses os.execvpe()-like behavior to execute the child program. On Windows, the class uses the Windows CreateProcess() function. The arguments to Popen are as follows.

args should be a sequence of program arguments or else a single string or path-like object. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.

警告

For maximum reliability, use a fully qualified path for the executable. To search for an unqualified name on PATH, use shutil.which(). On all platforms, passing sys.executable is the recommended way to launch the current Python interpreter again, and use the -m command-line format to launch an installed module.

Resolving the path of executable (or the first item of args) is platform dependent. For POSIX, see os.execvpe(), and note that when resolving or searching for the executable path, cwd overrides the current working directory and env can override the PATH environment variable. For Windows, see the documentation of the lpApplicationName and lpCommandLine parameters of WinAPI CreateProcess, and note that when resolving or searching for the executable path with shell=False, cwd does not override the current working directory and env cannot override the PATH environment variable. Using a full path avoids all of these variations.

An example of passing some arguments to an external program as a sequence is:

Popen(["/usr/bin/git", "commit", "-m", "Fixes a bug."])

On POSIX, if args is a string, the string is interpreted as the name or path of the program to execute. However, this can only be done if not passing arguments to the program.

備註

It may not be obvious how to break a shell command into a sequence of arguments, especially in complex cases. shlex.split() can illustrate how to determine the correct tokenization for args:

>>> import shlex, subprocess
>>> command_line = input()
/bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
>>> args = shlex.split(command_line)
>>> print(args)
['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
>>> p = subprocess.Popen(args) # 成功！

Note in particular that options (such as -input) and arguments (such as eggs.txt) that are separated by whitespace in the shell go in separate list elements, while arguments that need quoting or backslash escaping when used in the shell (such as filenames containing spaces or the echo command shown above) are single list elements.

On Windows, if args is a sequence, it will be converted to a string in a manner described in Converting an argument sequence to a string on Windows. This is because the underlying CreateProcess() operates on strings.

在 3.6 版的變更: args parameter accepts a path-like object if shell is False and a sequence containing path-like objects on POSIX.

在 3.8 版的變更: args parameter accepts a path-like object if shell is False and a sequence containing bytes and path-like objects on Windows.

The shell argument (which defaults to False) specifies whether to use the shell as the program to execute. If shell is True, it is recommended to pass args as a string rather than as a sequence.

On POSIX with shell=True, the shell defaults to /bin/sh. If args is a string, the string specifies the command to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say, Popen does the equivalent of:

Popen(['/bin/sh', '-c', args[0], args[1], ...])

On Windows with shell=True, the COMSPEC environment variable specifies the default shell. The only time you need to specify shell=True on Windows is when the command you wish to execute is built into the shell (e.g. dir or copy). You do not need shell=True to run a batch file or console-based executable.

備註

Read the Security Considerations section before using shell=True.

bufsize will be supplied as the corresponding argument to the open() function when creating the stdin/stdout/stderr pipe file objects:

  • 0 means unbuffered (read and write are one system call and can return short)

  • 1 means line buffered (only usable if text=True or universal_newlines=True)

  • any other positive value means use a buffer of approximately that size

  • negative bufsize (the default) means the system default of io.DEFAULT_BUFFER_SIZE will be used.

在 3.3.1 版的變更: bufsize now defaults to -1 to enable buffering by default to match the behavior that most code expects. In versions prior to Python 3.2.4 and 3.3.1 it incorrectly defaulted to 0 which was unbuffered and allowed short reads. This was unintentional and did not match the behavior of Python 2 as most code expected.

The executable argument specifies a replacement program to execute. It is very seldom needed. When shell=False, executable replaces the program to execute specified by args. However, the original args is still passed to the program. Most programs treat the program specified by args as the command name, which can then be different from the program actually executed. On POSIX, the args name becomes the display name for the executable in utilities such as ps. If shell=True, on POSIX the executable argument specifies a replacement shell for the default /bin/sh.

在 3.6 版的變更: executable parameter accepts a path-like object on POSIX.

在 3.8 版的變更: executable parameter accepts a bytes and path-like object on Windows.

在 3.12 版的變更: Changed Windows shell search order for shell=True. The current directory and %PATH% are replaced with %COMSPEC% and %SystemRoot%\System32\cmd.exe. As a result, dropping a malicious program named cmd.exe into a current directory no longer works.

stdin, stdout and stderr specify the executed program's standard input, standard output and standard error file handles, respectively. Valid values are None, PIPE, DEVNULL, an existing file descriptor (a positive integer), and an existing file object with a valid file descriptor. With the default settings of None, no redirection will occur. PIPE indicates that a new pipe to the child should be created. DEVNULL indicates that the special file os.devnull will be used. Additionally, stderr can be STDOUT, which indicates that the stderr data from the applications should be captured into the same file handle as for stdout.

If preexec_fn is set to a callable object, this object will be called in the child process just before the child is executed. (POSIX only)

警告

The preexec_fn parameter is NOT SAFE to use in the presence of threads in your application. The child process could deadlock before exec is called.

備註

If you need to modify the environment for the child use the env parameter rather than doing it in a preexec_fn. The start_new_session and process_group parameters should take the place of code using preexec_fn to call os.setsid() or os.setpgid() in the child.

在 3.8 版的變更: The preexec_fn parameter is no longer supported in subinterpreters. The use of the parameter in a subinterpreter raises RuntimeError. The new restriction may affect applications that are deployed in mod_wsgi, uWSGI, and other embedded environments.

If close_fds is true, all file descriptors except 0, 1 and 2 will be closed before the child process is executed. Otherwise when close_fds is false, file descriptors obey their inheritable flag as described in Inheritance of File Descriptors.

On Windows, if close_fds is true then no handles will be inherited by the child process unless explicitly passed in the handle_list element of STARTUPINFO.lpAttributeList, or by standard handle redirection.

在 3.2 版的變更: The default for close_fds was changed from False to what is described above.

在 3.7 版的變更: On Windows the default for close_fds was changed from False to True when redirecting the standard handles. It's now possible to set close_fds to True when redirecting the standard handles.

pass_fds is an optional sequence of file descriptors to keep open between the parent and child. Providing any pass_fds forces close_fds to be True. (POSIX only)

在 3.2 版的變更: 新增 pass_fds 參數。

If cwd is not None, the function changes the working directory to cwd before executing the child. cwd can be a string, bytes or path-like object. On POSIX, the function looks for executable (or for the first item in args) relative to cwd if the executable path is a relative path.

在 3.6 版的變更: cwd 參數在 POSIX 上接受 path-like object。

在 3.7 版的變更: cwd 參數在 Windows 上接受 path-like object。

在 3.8 版的變更: cwd 參數在 Windows 上接受 bytes 物件。

If restore_signals is true (the default) all signals that Python has set to SIG_IGN are restored to SIG_DFL in the child process before the exec. Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals. (POSIX only)

在 3.2 版的變更: 新增 restore_signals。

If start_new_session is true the setsid() system call will be made in the child process prior to the execution of the subprocess.

可用性: POSIX

在 3.2 版的變更: 新增 start_new_session。

If process_group is a non-negative integer, the setpgid(0, value) system call will be made in the child process prior to the execution of the subprocess.

可用性: POSIX

在 3.11 版的變更: 新增 process_group。

If group is not None, the setregid() system call will be made in the child process prior to the execution of the subprocess. If the provided value is a string, it will be looked up via grp.getgrnam() and the value in gr_gid will be used. If the value is an integer, it will be passed verbatim. (POSIX only)

可用性: POSIX

在 3.9 版被加入.

If extra_groups is not None, the setgroups() system call will be made in the child process prior to the execution of the subprocess. Strings provided in extra_groups will be looked up via grp.getgrnam() and the values in gr_gid will be used. Integer values will be passed verbatim. (POSIX only)

可用性: POSIX

在 3.9 版被加入.

If user is not None, the setreuid() system call will be made in the child process prior to the execution of the subprocess. If the provided value is a string, it will be looked up via pwd.getpwnam() and the value in pw_uid will be used. If the value is an integer, it will be passed verbatim. (POSIX only)

備註

Specifying user will not drop existing supplementary group memberships! The caller must also pass extra_groups=() to reduce the group membership of the child process for security purposes.

可用性: POSIX

在 3.9 版被加入.

If umask is not negative, the umask() system call will be made in the child process prior to the execution of the subprocess.

可用性: POSIX

在 3.9 版被加入.

If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process' environment. This mapping can be str to str on any platform or bytes to bytes on POSIX platforms much like os.environ or os.environb.

備註

If specified, env must provide any variables required for the program to execute. On Windows, in order to run a side-by-side assembly the specified env must include a valid %SystemRoot%.

If encoding or errors are specified, or text is true, the file objects stdin, stdout and stderr are opened in text mode with the specified encoding and errors, as described above in Frequently Used Arguments. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.

在 3.6 版被加入: 新增 encoding 與 errors。

在 3.7 版被加入: text was added as a more readable alias for universal_newlines.

If given, startupinfo will be a STARTUPINFO object, which is passed to the underlying CreateProcess function.

If given, creationflags, can be one or more of the following flags:

pipesize can be used to change the size of the pipe when PIPE is used for stdin, stdout or stderr. The size of the pipe is only changed on platforms that support this (only Linux at this time of writing). Other platforms will ignore this parameter.

在 3.10 版的變更: 新增 pipesize 參數。

Popen objects are supported as context managers via the with statement: on exit, standard file descriptors are closed, and the process is waited for.

with Popen(["ifconfig"], stdout=PIPE) as proc:
    log.write(proc.stdout.read())

Popen and the other functions in this module that use it raise an auditing event subprocess.Popen with arguments executable, args, cwd, and env. The value for args may be a single string or a list of strings, depending on platform.

在 3.2 版的變更: 新增情境管理器的支援。

在 3.6 版的變更: Popen destructor now emits a ResourceWarning warning if the child process is still running.

在 3.8 版的變更: Popen can use os.posix_spawn() in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, Popen constructor using os.posix_spawn() no longer raise an exception on errors like missing program, but the child process fails with a non-zero returncode.

例外

Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent.

The most common exception raised is OSError. This occurs, for example, when trying to execute a non-existent file. Applications should prepare for OSError exceptions. Note that, when shell=True, OSError will be raised by the child only if the selected shell itself was not found. To determine if the shell failed to find the requested application, it is necessary to check the return code or output from the subprocess.

A ValueError will be raised if Popen is called with invalid arguments.

check_call() and check_output() will raise CalledProcessError if the called process returns a non-zero return code.

All of the functions and methods that accept a timeout parameter, such as run() and Popen.communicate() will raise TimeoutExpired if the timeout expires before the process exits.

Exceptions defined in this module all inherit from SubprocessError.

在 3.3 版被加入: 新增 SubprocessError 基底類別。

安全性注意事項

Unlike some other popen functions, this library will not implicitly choose to call a system shell. This means that all characters, including shell metacharacters, can safely be passed to child processes. If the shell is invoked explicitly, via shell=True, it is the application's responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid shell injection vulnerabilities. On some platforms, it is possible to use shlex.quote() for this escaping.

On Windows, batch files (*.bat or *.cmd) may be launched by the operating system in a system shell regardless of the arguments passed to this library. This could result in arguments being parsed according to shell rules, but without any escaping added by Python. If you are intentionally launching a batch file with arguments from untrusted sources, consider passing shell=True to allow Python to escape special characters. See gh-114539 for additional discussion.

Popen Objects

Instances of the Popen class have the following methods:

Popen.poll()

Check if child process has terminated. Set and return returncode attribute. Otherwise, returns None.

Popen.wait(timeout=None)

Wait for child process to terminate. Set and return returncode attribute.

If the process does not terminate after timeout seconds, raise a TimeoutExpired exception. It is safe to catch this exception and retry the wait.

備註

This will deadlock when using stdout=PIPE or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use Popen.communicate() when using pipes to avoid that.

備註

When the timeout parameter is not None, then (on POSIX) the function is implemented using a busy loop (non-blocking call and short sleeps). Use the asyncio module for an asynchronous wait: see asyncio.create_subprocess_exec.

在 3.3 版的變更: 新增 timeout。

Popen.communicate(input=None, timeout=None)

Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate and set the returncode attribute. The optional input argument should be data to be sent to the child process, or None, if no data should be sent to the child. If streams were opened in text mode, input must be a string. Otherwise, it must be bytes.

communicate() returns a tuple