17.5. subprocess — Gerenciamento de subprocessos

Código Fonte: Lib/subprocess.py

O módulo subprocess permite que você crie novos processos, conecte-se aos seus canais de entrada/saída/erro e obtenha seus códigos de retorno. Este módulo pretende substituir vários módulos e funções mais antigos:

os.system
os.spawn*

Informações sobre como o módulo subprocess pode ser usado para substituir esses módulos e funções podem ser encontradas nas seções a seguir.

Ver também

PEP 324 – PEP propondo o módulo subprocess

17.5.1. Usando o módulo subprocess

A abordagem recomendada para invocar subprocessos é usar a função run() para todos os casos de uso que ela pode manipular. Para casos de uso mais avançados, a interface subjacente Popen pode ser usada diretamente.

A função run() foi adicionada no Python 3.5; se você precisa manter a compatibilidade com versões anteriores, veja a seção API de alto nível mais antiga.

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

Executa o comando descrito por args. Aguarda a conclusão do comando e retorna uma instância de CompletedProcess.

The arguments shown above are merely the most common ones, described below in Argumentos Usados Freqüentemente (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 - apart from timeout, input and check, all the arguments to this function are passed through to that interface.

This does not capture stdout or stderr by default. To do so, pass PIPE for the stdout and/or stderr arguments.

O argumento timeout é passado para Popen.communicate(). Se o tempo limite expirar, o processo filho será encerrado e aguardado. A exceção TimeoutExpired será levantada novamente após o término do processo filho.

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 universal_newlines is true. When used, the internal Popen object is automatically created with stdin=PIPE, and the stdin argument may not be used as well.

Se check for verdadeiro, e o processo sair com um código de saída diferente de zero, uma exceção CalledProcessError será levantada. Os atributos dessa exceção contêm os argumentos, o código de saída e stdout e stderr se eles foram capturados.

If encoding or errors are specified, or universal_newlines 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. Otherwise, file objects are opened in binary mode.

Se env não for None, deve ser um mapeamento que define as variáveis de ambiente para o novo processo; eles são usados em vez do comportamento padrão de herdar o ambiente do processo atual. É passado diretamente para Popen.

Exemplos:

>>> subprocess.run(["ls", "-l"])  # doesn't capture output
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"], stdout=subprocess.PIPE)
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')

Novo na versão 3.5.

Alterado na versão 3.6: Adicionado os parâmetros encoding e errors.

class subprocess.CompletedProcess

O valor de retorno de run(), representando um processo que foi concluído.

args

Os argumentos usados para iniciar o processo. Pode ser uma lista ou uma string.

returncode

Status de saída do processo filho. Normalmente, um status de saída 0 indica que foi executado com êxito.

Um valor negativo -N indica que o filho foi terminado pelo sinal N (POSIX apenas).

stdout

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

Se você executou o processo com stderr=subprocess.STDOUT, stdout e stderr serão combinados neste atributo, e stderr será None.

stderr

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

check_returncode()

Se returncode é diferente de zero, levantar um CalledProcessError.

Novo na versão 3.5.

subprocess.DEVNULL

Valor especial que pode ser usado como o argumento stdin, stdout ou stderr para Popen e indica que o arquivo especial os.devnull será usado.

Novo na versão 3.3.

subprocess.PIPE

Valor especial que pode ser usado como o argumento stdin, stdout ou stderr para Popen e indica que um encadeamento para o fluxo padrão deve ser aberto. Mais útil com Popen.communicate().

subprocess.STDOUT

Valor especial que pode ser usado como o argumento stderr para Popen e indica que o erro padrão deve ir para o mesmo manipulador que a saída padrão.

exception subprocess.SubprocessError

Classe base para todas as outras exceções deste módulo.

Novo na versão 3.3.

exception subprocess.TimeoutExpired

Subclasse de SubprocessError, levantada quando um tempo limite expira enquanto espera por um processo filho.

cmd

Comando que foi usado para gerar o processo filho.

timeout

Tempo limite em segundos.

output

Saída do processo filho se ele foi capturado por run() ou check_output(). Caso contrário, None.

stdout

Apelido para a saída, para simetria com stderr.

stderr

Saída Stderr do processo filho se ele foi capturado por run(). Caso contrário, None.

Novo na versão 3.3.

Alterado na versão 3.5: Atributos stdout e stderr adicionados

exception subprocess.CalledProcessError

Subclasse de SubprocessError, levantada quando um processo executado por check_call() ou check_output() retorna um status de saída diferente de zero.

returncode

Status de saída do processo filho. Se o processo foi encerrado devido a um sinal, este será o número do sinal negativo.

cmd

Comando que foi usado para gerar o processo filho.

output

Saída do processo filho se ele foi capturado por run() ou check_output(). Caso contrário, None.

stdout

Apelido para a saída, para simetria com stderr.

stderr

Saída Stderr do processo filho se ele foi capturado por run(). Caso contrário, None.

Alterado na versão 3.5: Atributos stdout e stderr adicionados

17.5.1.1. Argumentos Usados Freqüentemente

Para suportar uma ampla variedade de casos de uso, o construtor Popen (e as funções de conveniência) aceita muitos argumentos opcionais. Para a maioria dos casos de uso típicos, muitos desses argumentos podem ser seguramente deixados em seus valores padrão. Os argumentos mais comumente necessários são:

args é necessário para todas as chamadas e deve ser uma string ou uma sequência de argumentos do programa. Fornecer uma sequência de argumentos geralmente é preferível, pois permite que o módulo cuide de qualquer escapamento e citação de argumentos que forem necessários (por exemplo, para permitir espaços em nomes de arquivo). Se for passada uma única string, shell deve ser True (veja abaixo) ou então a string deve apenas nomear o programa a ser executado sem especificar nenhum argumento.

stdin, stdout e stderr especificam o manipulador de arquivo de entrada padrão, a saída padrão e de erro padrão do programa executado, respectivamente. Os valores válidos são PIPE, DEVNULL, um descritor de arquivo existente (um inteiro positivo), um objeto arquivo existente e None. PIPE indica que um novo encadeamento para o filho deve ser criado. DEVNULL indica que o arquivo especial os.devnull será usado. Com as configurações padrão de None, nenhum redirecionamento ocorrerá; os manipuladores de arquivo do filho serão herdados do pai. Além disso, stderr pode ser STDOUT, o que indica que os dados stderr do processo filho devem ser capturados no mesmo manipulador de arquivo que para stdout.

If encoding or errors are specified, or 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.

Para stdin, os caracteres de fim de linha '\n' na entrada serão convertidos para o separador de linha padrão os.linesep. Para stdout e stderr, todas as terminações de linha na saída serão convertidas para '\n'. Para obter mais informações, consulte a documentação da classe io.TextIOWrapper quando o argumento newline para seu construtor é None.

Se o modo de texto não for usado, stdin, stdout e stderr serão abertos como fluxos binários. Nenhuma codificação ou conversão de final de linha é executada.

Novo na versão 3.6: Adicionado os parâmetros encoding e errors.

Nota

O atributo newlines dos objetos arquivo Popen.stdin, Popen.stdout e Popen.stderr não são atualizados pelo método Popen.communicate().

Se shell for True, o comando especificado será executado através do shell. Isso pode ser útil se você estiver usando Python principalmente para o fluxo de controle aprimorado que ele oferece sobre a maioria dos shells do sistema e ainda deseja acesso conveniente a outros recursos do shell, como canais de shell, caracteres curinga de nome de arquivo, expansão de variável de ambiente e expansão de ~ para o diretório inicial de um usuário. No entanto, observe que o próprio Python oferece implementações de muitos recursos semelhantes a shell (em particular, glob, fnmatch, os.walk(), os.path.expandvars(), os.path.expanduser() e shutil).

Alterado na versão 3.3: Quando universal_newlines é True, a classe usa a codificação locale.getpreferredencoding(False) em vez de``locale.getpreferredencoding()``. Veja a classe io.TextIOWrapper para mais informações sobre esta alteração.

Nota

Leia a seção Security Considerations antes de usar shell=True.

Essas opções, junto com todas as outras opções, são descritas em mais detalhes na documentação do construtor Popen.

17.5.1.2. Construtor Popen

A criação e gerenciamento do processo subjacente neste módulo é manipulado pela classe Popen. Ela oferece muita flexibilidade para que os desenvolvedores sejam capazes de lidar com os casos menos comuns não cobertos pelas funções de conveniência.

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=False, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=(), *, encoding=None, errors=None)

Executa um programa filho em um novo processo. No POSIX, a classe usa o comportamento de os.execvp() para executar o programa filho. No Windows, a classe usa a função Windows CreateProcess(). Os argumentos para Popen são os seguintes.

args should be a sequence of program arguments or else a single string. 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.

No POSIX, se args for uma string, a string é interpretada como o nome ou caminho do programa a ser executado. No entanto, isso só pode ser feito se não forem passados argumentos para o programa.

Nota

shlex.split() can be useful when determining the correct tokenization for args, especially in complex cases:

>>> 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) # Success!

Observe em particular que as opções (como -input) e argumentos (como eggs.txt) que são separados por espaços em branco no shell vão em elementos de lista separados, enquanto os argumentos que precisam de aspas ou escape de contrabarra quando usados no shell (como nomes de arquivos contendo espaços ou o comando echo mostrado acima) são um único elemento de lista.

No Windows, se args for uma sequência, será convertido em uma string da maneira descrita em Converter uma sequência de argumentos em uma string no Windows. Isso ocorre porque o CreateProcess() subjacente opera em strings.

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.

Nota

Leia a seção Security Considerations antes de usar 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 universal_newlines=True i.e., in a text mode)

  • 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.

Alterado na versão 3.3.1: 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.

stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are PIPE, DEVNULL, an existing file descriptor (a positive integer), an existing file object, and None. PIPE indicates that a new pipe to the child should be created. DEVNULL indicates that the special file os.devnull will be used. With the default settings of None, no redirection will occur; the child’s file handles will be inherited from the parent. 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)

Aviso

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. If you must use it, keep it trivial! Minimize the number of libraries you call into.

Nota

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 parameter can take the place of a previously common use of preexec_fn to call os.setsid() in the child.

If close_fds is true, all file descriptors except 0, 1 and 2 will be closed before the child process is executed. (POSIX only). The default varies by platform: Always true on POSIX. On Windows it is true when stdin/stdout/stderr are None, false otherwise. On Windows, if close_fds is true then no handles will be inherited by the child process. Note that on Windows, you cannot set close_fds to true and also redirect the standard handles by setting stdin, stdout or stderr.

Alterado na versão 3.2: The default for close_fds was changed from False to what is described above.

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)

Novo na versão 3.2: O parâmetro pass_fds foi adicionado.

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

Alterado na versão 3.6: cwd parameter accepts a path-like object.

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)

Alterado na versão 3.2: restore_signals foi adicionado.

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

Alterado na versão 3.2: start_new_session foi adicionado.

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.

Nota

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, the file objects stdin, stdout and stderr are opened in text mode with the specified encoding and errors, as described above in Argumentos Usados Freqüentemente. If universal_newlines is True, they are opened in text mode with default encoding. Otherwise, they are opened as binary streams.

Novo na versão 3.6: encoding e errors foram adicionados.

If given, startupinfo will be a STARTUPINFO object, which is passed to the underlying CreateProcess function. creationflags, if given, can be CREATE_NEW_CONSOLE or CREATE_NEW_PROCESS_GROUP. (Windows only)

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())

Alterado na versão 3.2: Adicionado suporte a gerenciador de contexto.

Alterado na versão 3.6: O destruidor Popen agora emite um aviso ResourceWarning se o processo filho ainda estiver em execução.

17.5.1.3. Exceções

Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent. Additionally, the exception object will have one extra attribute called child_traceback, which is a string containing traceback information from the child’s point of view.

A exceção mais comum levantada é OSError. Isso ocorre, por exemplo, ao tentar executar um arquivo inexistente. Os aplicativos devem se preparar para exceções OSError.

A exceção ValueError será levantada se Popen for chamado com argumentos inválidos.

check_call() e check_output() levantarão CalledProcessError se o processo chamado retornar um código de retorno diferente de zero.

Todas as funções e métodos que aceitam um parâmetro de timeout, como call() e Popen.communicate() levantarão TimeoutExpired se o tempo limite expirar antes de o processo terminar.

Todas as exceções definidas neste módulo herdam de SubprocessError.

Novo na versão 3.3: A classe base SubprocessError foi adicionada.

17.5.2. Considerações de Segurança

Unlike some other popen functions, this implementation will never implicitly 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.

When using shell=True, the shlex.quote() function can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.

17.5.3. Objetos Popen

Instâncias da classe Popen têm os seguintes métodos:

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.

Nota

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.

Nota

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.

Alterado na versão 3.3: timeout foi adicionado.

Obsoleto desde a versão 3.4: Do not use the endtime parameter. It is was unintentionally exposed in 3.3 but was left undocumented as it was intended to be private for internal use. Use timeout instead.

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. 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 (stdout_data, stderr_data). The data will be strings if streams were opened in text mode; otherwise, bytes.

Note that if you want to send data to the process’s stdin, you need to create the Popen object with stdin=PIPE. Similarly, to get anything other than None in the result tuple, you need to give stdout=PIPE and/or stderr=PIPE too.

If the process does not terminate after timeout seconds, a TimeoutExpired exception will be raised. Catching this exception and retrying communication will not lose any output.

The child process is not killed if the timeout expires, so in order to cleanup properly a well-behaved application should kill the child process and finish communication:

proc = subprocess.Popen(...)
try:
    outs, errs = proc.communicate(timeout=15)
except TimeoutExpired:
    proc.kill()
    outs, errs = proc.communicate()

Nota

The data read is buffered in memory, so do not use this method if the data size is large or unlimited.

Alterado na versão 3.3: timeout foi adicionado.

Popen.send_signal(signal)

Envia o sinal signal para o filho.

Nota

On Windows, SIGTERM is an alias for terminate(). CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes started with a creationflags parameter which includes CREATE_NEW_PROCESS_GROUP.

Popen.terminate()

Stop the child. On Posix OSs the method sends SIGTERM to the child. On Windows the Win32 API function TerminateProcess() is called to stop the child.

Popen.kill()

Kills the child. On Posix OSs the function sends SIGKILL to the child. On Windows kill() is an alias for terminate().

Os seguintes atributos também estão disponíveis:

Popen.args

O argumento args conforme foi passado para Popen - uma sequência de argumentos do programa ou então uma única string.

Novo na versão 3.3.

Popen.stdin

If the stdin argument was PIPE, this attribute is a writeable stream object as returned by open(). If the encoding or errors arguments were specified or the universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stdin argument was not PIPE, this attribute is None.

Popen.stdout

If the stdout argument was PIPE, this attribute is a readable stream object as returned by open(). Reading from the stream provides output from the child process. If the encoding or errors arguments were specified or the universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stdout argument was not PIPE, this attribute is None.

Popen.stderr

If the stderr argument was PIPE, this attribute is a readable stream object as returned by open(). Reading from the stream provides error output from the child process. If the encoding or errors arguments were specified or the universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stderr argument was not PIPE, this attribute is None.

Aviso

Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

Popen.pid

O ID de processo do processo filho.

Observe que se você definir o argumento shell como True, este é o ID do processo do shell gerado.

Popen.returncode

The child return code, set by poll() and wait() (and indirectly by communicate()). A None value indicates that the process hasn’t terminated yet.

Um valor negativo -N indica que o filho foi terminado pelo sinal N (POSIX apenas).

17.5.4. Windows Popen Helpers

The STARTUPINFO class and following constants are only available on Windows.

class subprocess.STARTUPINFO

Partial support of the Windows STARTUPINFO structure is used for Popen creation.

dwFlags

A bit field that determines whether certain STARTUPINFO attributes are used when the process creates a window.

si = subprocess.STARTUPINFO()
si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW
hStdInput

If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard input handle for the process. If STARTF_USESTDHANDLES is not specified, the default for standard input is the keyboard buffer.

hStdOutput

If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard output handle for the process. Otherwise, this attribute is ignored and the default for standard output is the console window’s buffer.

hStdError

If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard error handle for the process. Otherwise, this attribute is ignored and the default for standard error is the console window’s buffer.

wShowWindow

If dwFlags specifies STARTF_USESHOWWINDOW, this attribute can be any of the values that can be specified in the nCmdShow parameter for the ShowWindow function, except for SW_SHOWDEFAULT. Otherwise, this attribute is ignored.

SW_HIDE is provided for this attribute. It is used when Popen is called with shell=True.

17.5.4.1. Constantes

The subprocess module exposes the following constants.

subprocess.STD_INPUT_HANDLE

The standard input device. Initially, this is the console input buffer, CONIN$.

subprocess.STD_OUTPUT_HANDLE

The standard output device. Initially, this is the active console screen buffer, CONOUT$.

subprocess.STD_ERROR_HANDLE

The standard error device. Initially, this is the active console screen buffer, CONOUT$.

subprocess.SW_HIDE

Oculta a janela. Outra janela será ativada.

subprocess.STARTF_USESTDHANDLES

Specifies that the STARTUPINFO.hStdInput, STARTUPINFO.hStdOutput, and STARTUPINFO.hStdError attributes contain additional information.

subprocess.STARTF_USESHOWWINDOW

Specifies that the STARTUPINFO.wShowWindow attribute contains additional information.

subprocess.CREATE_NEW_CONSOLE

The new process has a new console, instead of inheriting its parent’s console (the default).

subprocess.CREATE_NEW_PROCESS_GROUP

A Popen creationflags parameter to specify that a new process group will be created. This flag is necessary for using os.kill() on the subprocess.

This flag is ignored if CREATE_NEW_CONSOLE is specified.

17.5.5. API de alto nível mais antiga

Antes do Python 3.5, essas três funções constituíam a API de alto nível para subprocesso. Agora você pode usar run() em muitos casos, mas muitos códigos existentes chamam essas funções.

subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)

Run the command described by args. Wait for command to complete, then return the returncode attribute.

This is equivalent to:

run(...).returncode

(except that the input and check parameters are not supported)

The arguments shown above are merely the most common ones. The full function signature is largely the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.

Nota

Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.

Alterado na versão 3.3: timeout foi adicionado.

subprocess.check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)

Run command with arguments. Wait for command to complete. If the return code was zero then return, otherwise raise CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute.

This is equivalent to:

run(..., check=True)

(except that the input parameter is not supported)

The arguments shown above are merely the most common ones. The full function signature is largely the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.

Nota

Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.

Alterado na versão 3.3: timeout foi adicionado.

subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, cwd=None, encoding=None, errors=None, universal_newlines=False, timeout=None)

Executa o comando com argumentos e retorna sua saída.

If the return code was non-zero it raises a CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute and any output in the output attribute.

This is equivalent to:

run(..., check=True, stdout=PIPE).stdout

The arguments shown above are merely the most common ones. The full function signature is largely the same as that of run() - most arguments are passed directly through to that interface. However, explicitly passing input=None to inherit the parent’s standard input file handle is not supported.

By default, this function will return the data as encoded bytes. The actual encoding of the output data may depend on the command being invoked, so the decoding to text will often need to be handled at the application level.

This behaviour may be overridden by setting universal_newlines to True as described above in Argumentos Usados Freqüentemente.

To also capture standard error in the result, use stderr=subprocess.STDOUT:

>>> subprocess.check_output(
...     "ls non_existent_file; exit 0",
...     stderr=subprocess.STDOUT,
...     shell=True)
'ls: non_existent_file: No such file or directory\n'

Novo na versão 3.1.

Alterado na versão 3.3: timeout foi adicionado.

Alterado na versão 3.4: Support for the input keyword argument was added.

Alterado na versão 3.6: encoding and errors were added. See run() for details.

17.5.6. Replacing Older Functions with the subprocess Module

In this section, “a becomes b” means that b can be used as a replacement for a.

Nota

All “a” functions in this section fail (more or less) silently if the executed program cannot be found; the “b” replacements raise OSError instead.

In addition, the replacements using check_output() will fail with a CalledProcessError if the requested operation produces a non-zero return code. The output is still available as the output attribute of the raised exception.

In the following examples, we assume that the relevant functions have already been imported from the subprocess module.

17.5.6.1. Replacing /bin/sh shell backquote

output=`mycmd myarg`

torna-se:

output = check_output(["mycmd", "myarg"])

17.5.6.2. Replacing shell pipeline

output=`dmesg | grep hda`

torna-se:

p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]

The p1.stdout.close() call after starting the p2 is important in order for p1 to receive a SIGPIPE if p2 exits before p1.

Alternatively, for trusted input, the shell’s own pipeline support may still be used directly:

output=`dmesg | grep hda`

torna-se:

output=check_output("dmesg | grep hda", shell=True)

17.5.6.3. Substituindo os.system()

sts = os.system("mycmd" + " myarg")
# becomes
sts = call("mycmd" + " myarg", shell=True)

Notas:

  • Calling the program through the shell is usually not required.

Um exemplo mais realista ficaria assim:

try:
    retcode = call("mycmd" + " myarg", shell=True)
    if retcode < 0:
        print("Child was terminated by signal", -retcode, file=sys.stderr)
    else:
        print("Child returned", retcode, file=sys.stderr)
except OSError as e:
    print("Execution failed:", e, file=sys.stderr)

17.5.6.4. Replacing the os.spawn family

Exemplo P_NOWAIT:

pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid

Exemplo P_WAIT:

retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])

Exemplo de vetor:

os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])

Exemplo de ambiente:

os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})

17.5.6.5. Replacing os.popen(), os.popen2(), os.popen3()

(child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdin, child_stdout) = (p.stdin, p.stdout)

(child_stdin,
 child_stdout,
 child_stderr) = os.popen3(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
(child_stdin,
 child_stdout,
 child_stderr) = (p.stdin, p.stdout, p.stderr)

(child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)

Return code handling translates as follows:

pipe = os.popen(cmd, 'w')
...
rc = pipe.close()
if rc is not None and rc >> 8:
    print("There were some errors")
==>
process = Popen(cmd, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
    print("There were some errors")

17.5.6.6. Replacing functions from the popen2 module

Nota

If the cmd argument to popen2 functions is a string, the command is executed through /bin/sh. If it is a list, the command is directly executed.

(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
==>
p = Popen("somestring", shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)

(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)

popen2.Popen3 and popen2.Popen4 basically work as subprocess.Popen, except that:

  • Popen raises an exception if the execution fails.

  • the capturestderr argument is replaced with the stderr argument.

  • stdin=PIPE and stdout=PIPE must be specified.

  • popen2 closes all file descriptors by default, but you have to specify close_fds=True with Popen to guarantee this behavior on all platforms or past Python versions.

17.5.7. Legacy Shell Invocation Functions

This module also provides the following legacy functions from the 2.x commands module. These operations implicitly invoke the system shell and none of the guarantees described above regarding security and exception handling consistency are valid for these functions.

subprocess.getstatusoutput(cmd)

Return (exitcode, output) of executing cmd in a shell.

Execute the string cmd in a shell with Popen.check_output() and return a 2-tuple (exitcode, output). The locale encoding is used; see the notes on Argumentos Usados Freqüentemente for more details.

A trailing newline is stripped from the output. The exit code for the command can be interpreted as the return code of subprocess. Example:

>>> subprocess.getstatusoutput('ls /bin/ls')
(0, '/bin/ls')
>>> subprocess.getstatusoutput('cat /bin/junk')
(1, 'cat: /bin/junk: No such file or directory')
>>> subprocess.getstatusoutput('/bin/junk')
(127, 'sh: /bin/junk: not found')
>>> subprocess.getstatusoutput('/bin/kill $$')
(-15, '')

Availability: POSIX & Windows

Alterado na versão 3.3.4: Suporte ao Windows foi adicionado.

The function now returns (exitcode, output) instead of (status, output) as it did in Python 3.3.3 and earlier. exitcode has the same value as returncode.

subprocess.getoutput(cmd)

Return output (stdout and stderr) of executing cmd in a shell.

Like getstatusoutput(), except the exit code is ignored and the return value is a string containing the command’s output. Example:

>>> subprocess.getoutput('ls /bin/ls')
'/bin/ls'

Availability: POSIX & Windows

Alterado na versão 3.3.4: Suporte para Windows adicionado.

17.5.8. Notas

17.5.8.1. Converter uma sequência de argumentos em uma string no Windows

On Windows, an args sequence is converted to a string that can be parsed using the following rules (which correspond to the rules used by the MS C runtime):

  1. Arguments are delimited by white space, which is either a space or a tab.

  2. A string surrounded by double quotation marks is interpreted as a single argument, regardless of white space contained within. A quoted string can be embedded in an argument.

  3. A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark.

  4. Backslashes are interpreted literally, unless they immediately precede a double quotation mark.

  5. If backslashes immediately precede a double quotation mark, every pair of backslashes is interpreted as a literal backslash. If the number of backslashes is odd, the last backslash escapes the next double quotation mark as described in rule 3.

Ver também

shlex

Module which provides function to parse and escape command lines.