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annotate Doc/lib/libsubprocess.tex @ 33845:032f342efcd2 legacy-trunk

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Very few little improvements.
author Facundo Batista <facundobatista@gmail.com>
date Tue, 26 Oct 2004 23:38:46 +0000
parents fbacbb262789
children 0ae28c3a8d24
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1 \section{\module{subprocess} --- Subprocess management}
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2
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3 \declaremodule{standard}{subprocess}
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4 \modulesynopsis{Subprocess management.}
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5 \moduleauthor{Peter \AA strand}{astrand@lysator.liu.se}
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6 \sectionauthor{Peter \AA strand}{astrand@lysator.liu.se}
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7
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8 \versionadded{2.4}
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9
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10 The \module{subprocess} module allows you to spawn new processes,
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11 connect to their input/output/error pipes, and obtain their return
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12 codes. This module intends to replace several other, older modules
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13 and functions, such as:
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14
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15 % XXX Should add pointers to this module to at least the popen2
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16 % and commands sections.
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17
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18 \begin{verbatim}
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19 os.system
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20 os.spawn*
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21 os.popen*
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22 popen2.*
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23 commands.*
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24 \end{verbatim}
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25
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26 Information about how the \module{subprocess} module can be used to
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27 replace these modules and functions can be found in the following
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28 sections.
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29
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30 \subsection{Using the subprocess Module}
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31
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32 This module defines one class called \class{Popen}:
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33
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34 \begin{classdesc}{Popen}{args, bufsize=0, executable=None,
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35 stdin=None, stdout=None, stderr=None,
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36 preexec_fn=None, close_fds=False, shell=False,
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37 cwd=None, env=None, universal_newlines=False,
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38 startupinfo=None, creationflags=0}
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39
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40 Arguments are:
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41
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42 \var{args} should be a string, or a sequence of program arguments. The
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43 program to execute is normally the first item in the args sequence or
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44 string, but can be explicitly set by using the executable argument.
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45
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46 On \UNIX{}, with \var{shell=False} (default): In this case, the Popen
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47 class uses \method{os.execvp()} to execute the child program.
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48 \var{args} should normally be a sequence. A string will be treated as a
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49 sequence with the string as the only item (the program to execute).
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50
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51 On \UNIX{}, with \var{shell=True}: If args is a string, it specifies the
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52 command string to execute through the shell. If \var{args} is a
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53 sequence, the first item specifies the command string, and any
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54 additional items will be treated as additional shell arguments.
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55
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56 On Windows: the \class{Popen} class uses CreateProcess() to execute
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57 the child program, which operates on strings. If \var{args} is a
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58 sequence, it will be converted to a string using the
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59 \method{list2cmdline} method. Please note that not all MS Windows
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60 applications interpret the command line the same way:
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61 \method{list2cmdline} is designed for applications using the same
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62 rules as the MS C runtime.
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63
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64 \var{bufsize}, if given, has the same meaning as the corresponding
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65 argument to the built-in open() function: \constant{0} means unbuffered,
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66 \constant{1} means line buffered, any other positive value means use a
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67 buffer of (approximately) that size. A negative \var{bufsize} means to
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68 use the system default, which usually means fully buffered. The default
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69 value for \var{bufsize} is \constant{0} (unbuffered).
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70
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71 \var{stdin}, \var{stdout} and \var{stderr} specify the executed
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72 programs' standard input, standard output and standard error file
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73 handles, respectively. Valid values are \code{PIPE}, an existing file
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74 descriptor (a positive integer), an existing file object, and
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75 \code{None}. \code{PIPE} indicates that a new pipe to the child
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76 should be created. With \code{None}, no redirection will occur; the
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77 child's file handles will be inherited from the parent. Additionally,
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78 \var{stderr} can be \code{STDOUT}, which indicates that the stderr
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79 data from the applications should be captured into the same file
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80 handle as for stdout.
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81
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82 If \var{preexec_fn} is set to a callable object, this object will be
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83 called in the child process just before the child is executed.
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84
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85 If \var{close_fds} is true, all file descriptors except \constant{0},
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86 \constant{1} and \constant{2} will be closed before the child process is
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87 executed.
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88
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89 If \var{shell} is \constant{True}, the specified command will be
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90 executed through the shell.
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91
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92 If \var{cwd} is not \code{None}, the current directory will be changed
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93 to cwd before the child is executed.
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94
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95 If \var{env} is not \code{None}, it defines the environment variables
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96 for the new process.
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97
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98 If \var{universal_newlines} is \constant{True}, the file objects stdout
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99 and stderr are opened as a text files, but lines may be terminated by
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100 any of \code{'\e n'}, the Unix end-of-line convention, \code{'\e r'},
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101 the Macintosh convention or \code{'\e r\e n'}, the Windows convention.
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102 All of these external representations are seen as \code{'\e n'} by the
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103 Python program. \note{This feature is only available if Python is built
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104 with universal newline support (the default). Also, the newlines
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105 attribute of the file objects \member{stdout}, \member{stdin} and
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106 \member{stderr} are not updated by the communicate() method.}
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107
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108 The \var{startupinfo} and \var{creationflags}, if given, will be
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109 passed to the underlying CreateProcess() function. They can specify
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110 things such as appearance of the main window and priority for the new
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111 process. (Windows only)
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112 \end{classdesc}
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113
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114 \subsubsection{Convenience Functions}
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115
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116 This module also defines one shortcut function:
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117
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118 \begin{funcdesc}{call}{*args, **kwargs}
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119 Run command with arguments. Wait for command to complete, then
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120 return the \member{returncode} attribute.
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121
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122 The arguments are the same as for the Popen constructor. Example:
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123
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124 \begin{verbatim}
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125 retcode = call(["ls", "-l"])
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126 \end{verbatim}
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127 \end{funcdesc}
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128
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129
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130 \subsubsection{Exceptions}
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131
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132 Exceptions raised in the child process, before the new program has
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133 started to execute, will be re-raised in the parent. Additionally,
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134 the exception object will have one extra attribute called
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135 \member{child_traceback}, which is a string containing traceback
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136 information from the childs point of view.
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137
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138 The most common exception raised is \exception{OSError}. This occurs,
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139 for example, when trying to execute a non-existent file. Applications
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140 should prepare for \exception{OSError} exceptions.
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141
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142 A \exception{ValueError} will be raised if \class{Popen} is called
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143 with invalid arguments.
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144
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145
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146 \subsubsection{Security}
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147
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148 Unlike some other popen functions, this implementation will never call
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149 /bin/sh implicitly. This means that all characters, including shell
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150 metacharacters, can safely be passed to child processes.
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151
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152
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153 \subsection{Popen Objects}
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154
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155 Instances of the \class{Popen} class have the following methods:
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156
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157 \begin{methoddesc}{poll}{}
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158 Check if child process has terminated. Returns returncode
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159 attribute.
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160 \end{methoddesc}
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161
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162 \begin{methoddesc}{wait}{}
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163 Wait for child process to terminate. Returns returncode attribute.
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164 \end{methoddesc}
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165
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166 \begin{methoddesc}{communicate}{input=None}
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167 Interact with process: Send data to stdin. Read data from stdout and
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168 stderr, until end-of-file is reached. Wait for process to terminate.
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169 The optional \var{stdin} argument should be a string to be sent to the
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170 child process, or \code{None}, if no data should be sent to the child.
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171
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172 communicate() returns a tuple (stdout, stderr).
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173
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174 \note{The data read is buffered in memory, so do not use this method
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175 if the data size is large or unlimited.}
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176 \end{methoddesc}
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177
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178 The following attributes are also available:
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179
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180 \begin{memberdesc}{stdin}
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181 If the \var{stdin} argument is \code{PIPE}, this attribute is a file
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182 object that provides input to the child process. Otherwise, it is
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183 \code{None}.
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184 \end{memberdesc}
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185
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186 \begin{memberdesc}{stdout}
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187 If the \var{stdout} argument is \code{PIPE}, this attribute is a file
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188 object that provides output from the child process. Otherwise, it is
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189 \code{None}.
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190 \end{memberdesc}
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191
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192 \begin{memberdesc}{stderr}
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193 If the \var{stderr} argument is \code{PIPE}, this attribute is file
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194 object that provides error output from the child process. Otherwise,
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195 it is \code{None}.
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196 \end{memberdesc}
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197
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198 \begin{memberdesc}{pid}
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199 The process ID of the child process.
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200 \end{memberdesc}
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201
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202 \begin{memberdesc}{returncode}
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203 The child return code. A \code{None} value indicates that the process
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204 hasn't terminated yet. A negative value -N indicates that the child
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205 was terminated by signal N (\UNIX{} only).
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206 \end{memberdesc}
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207
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208
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209 \subsection{Replacing Older Functions with the subprocess Module}
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210
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211 In this section, "a ==> b" means that b can be used as a replacement
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212 for a.
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213
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214 \note{All functions in this section fail (more or less) silently if
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215 the executed program cannot be found; this module raises an
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216 \exception{OSError} exception.}
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217
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218 In the following examples, we assume that the subprocess module is
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219 imported with "from subprocess import *".
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220
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221 \subsubsection{Replacing /bin/sh shell backquote}
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222
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223 \begin{verbatim}
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224 output=`mycmd myarg`
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225 ==>
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226 output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
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227 \end{verbatim}
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228
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229 \subsubsection{Replacing shell pipe line}
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230
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231 \begin{verbatim}
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232 output=`dmesg | grep hda`
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233 ==>
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234 p1 = Popen(["dmesg"], stdout=PIPE)
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235 p2 = Popen(["grep", "hda"], stdin=p1.stdout)
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236 output = p2.communicate()[0]
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237 \end{verbatim}
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238
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239 \subsubsection{Replacing os.system()}
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240
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241 \begin{verbatim}
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242 sts = os.system("mycmd" + " myarg")
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243 ==>
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244 p = Popen("mycmd" + " myarg", shell=True)
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245 sts = os.waitpid(p.pid, 0)
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246 \end{verbatim}
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247
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248 Notes:
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249
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250 \begin{itemize}
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251 \item Calling the program through the shell is usually not required.
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252 \item It's easier to look at the \member{returncode} attribute than
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253 the exit status.
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254 \end{itemize}
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255
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256 A more realistic example would look like this:
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257
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258 \begin{verbatim}
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259 try:
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260 retcode = call("mycmd" + " myarg", shell=True)
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261 if retcode < 0:
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262 print >>sys.stderr, "Child was terminated by signal", -retcode
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263 else:
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264 print >>sys.stderr, "Child returned", retcode
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265 except OSError, e:
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266 print >>sys.stderr, "Execution failed:", e
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267 \end{verbatim}
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268
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269 \subsubsection{Replacing os.spawn*}
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270
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271 P_NOWAIT example:
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272
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273 \begin{verbatim}
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274 pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
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275 ==>
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276 pid = Popen(["/bin/mycmd", "myarg"]).pid
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277 \end{verbatim}
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278
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279 P_WAIT example:
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280
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281 \begin{verbatim}
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282 retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
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283 ==>
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284 retcode = call(["/bin/mycmd", "myarg"])
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285 \end{verbatim}
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286
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287 Vector example:
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288
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289 \begin{verbatim}
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290 os.spawnvp(os.P_NOWAIT, path, args)
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291 ==>
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292 Popen([path] + args[1:])
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293 \end{verbatim}
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294
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295 Environment example:
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296
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297 \begin{verbatim}
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298 os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
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299 ==>
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300 Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})
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301 \end{verbatim}
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302
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303 \subsubsection{Replacing os.popen*}
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304
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305 \begin{verbatim}
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306 pipe = os.popen(cmd, mode='r', bufsize)
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307 ==>
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308 pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout
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309 \end{verbatim}
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310
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311 \begin{verbatim}
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312 pipe = os.popen(cmd, mode='w', bufsize)
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313 ==>
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314 pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin
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315 \end{verbatim}
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316
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317 \begin{verbatim}
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318 (child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
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319 ==>
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320 p = Popen(cmd, shell=True, bufsize=bufsize,
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321 stdin=PIPE, stdout=PIPE, close_fds=True)
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322 (child_stdin, child_stdout) = (p.stdin, p.stdout)
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323 \end{verbatim}
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324
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325 \begin{verbatim}
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326 (child_stdin,
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327 child_stdout,
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328 child_stderr) = os.popen3(cmd, mode, bufsize)
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329 ==>
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330 p = Popen(cmd, shell=True, bufsize=bufsize,
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331 stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
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332 (child_stdin,
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333 child_stdout,
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334 child_stderr) = (p.stdin, p.stdout, p.stderr)
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335 \end{verbatim}
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336
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337 \begin{verbatim}
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338 (child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
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339 ==>
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340 p = Popen(cmd, shell=True, bufsize=bufsize,
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341 stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
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342 (child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)
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343 \end{verbatim}
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344
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345 \subsubsection{Replacing popen2.*}
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346
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347 \note{If the cmd argument to popen2 functions is a string, the command
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348 is executed through /bin/sh. If it is a list, the command is directly
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349 executed.}
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350
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351 \begin{verbatim}
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352 (child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
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353 ==>
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354 p = Popen(["somestring"], shell=True, bufsize=bufsize
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355 stdin=PIPE, stdout=PIPE, close_fds=True)
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356 (child_stdout, child_stdin) = (p.stdout, p.stdin)
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357 \end{verbatim}
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358
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359 \begin{verbatim}
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360 (child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
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361 ==>
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362 p = Popen(["mycmd", "myarg"], bufsize=bufsize,
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363 stdin=PIPE, stdout=PIPE, close_fds=True)
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364 (child_stdout, child_stdin) = (p.stdout, p.stdin)
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365 \end{verbatim}
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366
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367 The popen2.Popen3 and popen3.Popen4 basically works as subprocess.Popen,
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368 except that:
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369
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370 \begin{itemize}
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371 \item subprocess.Popen raises an exception if the execution fails
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372
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373 \item the \var{capturestderr} argument is replaced with the \var{stderr}
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374 argument.
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375
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376 \item stdin=PIPE and stdout=PIPE must be specified.
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377
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378 \item popen2 closes all file descriptors by default, but you have to
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379 specify close_fds=True with subprocess.Popen.
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380 \end{itemize}