PEP 787: Safer subprocess usage using t-strings

.github/CODEOWNERS

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 peps/pep-0783.rst  @hoodmane @ambv
 peps/pep-0784.rst  @gpshead
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+peps/pep-0787.rst  @ncoghlan
+# ...
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peps/pep-0787.rst

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+PEP: 787
+Title: Safer subprocess usage using t-strings
+Author: Nick Humrich <nick@humrich.us>, Alyssa Coghlan <ncoghlan@gmail.com>
+Discussions-To: Pending
+Status: Draft
+Type: Standards Track
+Requires: 750
+Created: 13-Apr-2025
+Python-Version: 3.14
+
+Abstract
+========
+
+:pep:`750` introduced template strings (t-strings) as a generalization of f-strings,
+providing a way to safely handle string interpolation in various contexts. This PEP
+proposes extending the :mod:`subprocess` and :mod:`shlex` modules to natively support t-strings, enabling
+safer and more ergonomic shell command execution with interpolated values, as well as
+serving as a reference implementation for the t-string feature to improve API ergonomics.
+
+Motivation
+==========
+
+Despite the safety benefits and flexibility that template strings offer in PEP 750,
+they lack a concrete consumer implementation in the standard library that demonstrates
+their practical application. One of the most compelling use cases for t-strings is safer
+shell command execution, as noted in the withdrawn :pep:`501`:
+
+.. code-block:: python
+
+    # Unsafe with f-strings:
+    os.system(f"echo {message_from_user}")
+
+    # Also unsafe with f-strings
+    subprocess.run(f"echo {message_from_user}", shell=True)
+
+    # Fails with f-strings
+    subprocess.run(f"echo {message_from_user}")
+
+    # Safe with t-strings and POSIX-compliant shell quoting:
+    subprocess.run(t"echo {message_from_user}", shell=True)
+
+    # Safe on all platforms with t-strings:
+    subprocess.run(t"echo {message_from_user}")
+
+    # Safe on all platforms without t-strings:
+    subprocess.run(["echo", str(message_from_user)])
+
+Currently, developers must choose between convenience (using f-strings with potential
+security risks) and safety (using more verbose, list-based APIs). By adding native t-string
+support to the :mod:`subprocess` module, we provide a consumer reference implementation that
+demonstrates the value of t-strings while addressing a common security concern.
+
+Rationale
+=========
+
+The subprocess module is an ideal candidate for t-string support for several reasons:
+
+* Command injection vulnerabilities in shell commands are a well-known security risk.
+* The :mod:`subprocess` module already supports both string and list-based command specifications.
+* There's a natural mapping between t-strings and proper shell escaping that provides both convenience and safety.
+* It serves as a practical showcase for t-strings that developers can immediately understand and appreciate.
+
+By extending subprocess to handle t-strings natively, we make it easier to write secure code without sacrificing
+the convenience that led many developers to use potentially unsafe f-strings.
+
+Specification
+=============
+
+This PEP proposes two main additions to the standard library:
+
+#. A new ``sh()`` renderer function in the :mod:`shlex` module for safe shell command construction
+#. Adding t-string support to the :mod:`subprocess` module's core functions,
+    particularly :class:`subprocess.Popen`, :func:`subprocess.run`, and other related functions
+    that accept a command argument
+
+
+Renderer for shell escaping added to :mod:`shlex`
+-------------------------------------------------
+
+As a reference implementation, a renderer for safe POSIX shell escaping will be added to
+the :mod:`shlex` module. This renderer would be called ``sh`` and would be equivalent to
+calling ``shlex.quote`` on each field value in the template literal.
+
+Thus::
+
+  os.system(shlex.sh(t"cat {myfile}"))
+
+would have the same behavior as::
+
+  os.system("cat " + shlex.quote(myfile)))
+
+
+The addition of ``shlex.sh`` will NOT change the existing admonishments in the
+:mod:`subprocess` documentation that passing ``shell=True`` is best avoided, nor the
+reference from the :func:`os.system` documentation to the higher level ``subprocess`` APIs.
+
+The t-string processor implementation would look like::
+
+    from string.templatelib import Template, Interpolation
+
+    def sh(template: Template) -> str:
+        parts: list[str] = []
+        for item in template:
+            if isinstance(item, Interpolation):
+                # shlex.sh implementation, so shlex.quote can be used directly
+                parts.append(quote(str(item.value)))
+            else:
+                parts.append(item)
+
+        # shlex.sh implementation, so `join` references shlex.join
+        return join(parts)
+
+This allows for explicit escaping of t-strings for shell usage::
+
+    import shlex
+    # Safe POSIX-compliant shell command construction
+    command = shlex.sh(t"cat {filename}")
+    os.system(command)
+
+Changes to subprocess module
+----------------------------
+
+With the additional renderer in the shlex module, and the addition of template strings,
+the :mod:`subprocess` module can be changed to handle accepting template strings
+as an additional input type to ``Popen``, as it already accepts a sequence, or a string,
+with different behavior for each. In return, all :class:`subprocess.Popen` higher level
+functions such as :func:`subprocess.run` could accept strings in a safe way
+(on all systems for ``shell=False`` and on :ref:`POSIX systems <pep-0787-defer-non-posix-shells>` for ``shell=True``).
+
+For example::
+
+  subprocess.run(t"cat {myfile}", shell=True)
+
+would automatically use the ``shlex.sh`` renderer provided in this PEP. Therefore, using
+``shlex`` inside a ``subprocess.run`` call like so::
+
+  subprocess.run(shlex.sh(t"cat {myfile}"), shell=True)
+
+would be redundant, as ``run`` would automatically render any template literals
+through ``shlex.sh``
+
+When ``subprocess.Popen`` is called without ``shell=True``, t-string support would still
+provide subprocess with a more ergonomic syntax. For example::
+
+  subprocess.run(t"cat {myfile} --flag {value}")
+
+would be equivalent to::
+
+  subprocess.run(["cat", myfile, "--flag", value])
+
+or, more accurately::
+
+  subprocess.run(shlex.split(f"cat {shlex.quote(myfile)} --flag {shlex.quote(value)}"))
+
+It would do this by first using the ``shlex.sh`` renderer, as above, then using
+``shlex.split`` on the result.
+
+The implementation inside ``subprocess.Popen._execute_child`` would check for t-strings::
+
+    from string.templatelib import Template
+
+    if isinstance(args, Template):
+        import shlex
+        if shell:
+            args = shlex.sh(args)
+        else:
+            args = shlex.split(shlex.sh(args))
+
+Backwards Compatibility
+=======================
+
+This change is fully backwards compatible as it only adds new functionality without altering existing behavior.
+The subprocess module will continue to handle strings and lists in the same way it currently does.
+
+Security Implications
+=====================
+
+This PEP is explicitly designed to improve security by providing a safer alternative to using
+f-strings with shell commands. By automatically applying appropriate escaping based on context
+(shell or non-shell), it helps prevent command injection vulnerabilities.
+
+However, it's worth noting that when ``shell=True`` is used, the safety is limited to
+POSIX-compliant shells. On Windows systems where cmd.exe or PowerShell may be used as the shell,
+the escaping mechanism provided by :func:`shlex.quote` is not sufficient to prevent all forms
+of command injection.
+
+How to Teach This
+=================
+
+This feature can be taught as a natural extension of t-strings that demonstrates their practical value:
+
+1. Introduce the problem of command injection and why f-strings are dangerous with shell commands
+2. Show the traditional solutions (list-based commands, manual escaping)
+3. Introduce the ``shlex.sh`` renderer for explicit shell escaping::
+
+       # Unsafe:
+       os.system(f"cat {filename}")  # Potential command injection!
+
+       # Safe using shlex.sh:
+       os.system(shlex.sh(t"cat {filename}"))  # Explicitly escaping for shell
+
+4. Introduce the subprocess module's native t-string support::
+
+       # Unsafe:
+       subprocess.run(f"cat {filename}", shell=True)  # Potential command injection!
+
+       # Safe but verbose:
+       subprocess.run(["cat", filename])
+
+       # Safe and readable with t-strings:
+       subprocess.run(t"cat {filename}", shell=True)  # Automatically escapes filename
+       subprocess.run(t"cat {filename}")  # Automatically converts to list form
+
+The implementation should be added to both the shlex and subprocess module documentation with clear
+examples and security advisories.
+
+.. _pep-0787-defer-non-posix-shells:
+
+Deferring escaped rendering support for non-POSIX shells
+--------------------------------------------------------
+
+:func:`shlex.quote` works by classifying the regex character set ``[\w@%+=:,./-]`` to be
+safe, deeming all other characters to be unsafe, and hence requiring quoting of the string
+containing them. The quoting mechanism used is then specific to the way that string quoting
+works in POSIX shells, so it cannot be trusted when running a shell that doesn't follow
+POSIX shell string quoting rules.
+
+For example, running ``subprocess.run(f"echo {shlex.quote(sys.argv[1])}", shell=True)`` is
+safe when using a shell that follows POSIX quoting rules:
+
+.. code-block:: console
+
+    $ cat > run_quoted.py
+    import sys, shlex, subprocess
+    subprocess.run(f"echo {shlex.quote(sys.argv[1])}", shell=True)
+    $ python3 run_quoted.py pwd
+    pwd
+    $ python3 run_quoted.py '; pwd'
+    ; pwd
+    $ python3 run_quoted.py "'pwd'"
+    'pwd'
+
+but remains unsafe when running a shell from Python invokes ``cmd.exe`` (or Powershell):
+
+.. code-block:: powershell
+
+    S:\> echo import sys, shlex, subprocess > run_quoted.py
+    S:\> echo subprocess.run(f"echo {shlex.quote(sys.argv[1])}", shell=True) >> run_quoted.py
+    S:\> type run_quoted.py
+    import sys, shlex, subprocess
+    subprocess.run(f"echo {shlex.quote(sys.argv[1])}", shell=True)
+    S:\> python3 run_quoted.py "echo OK"
+    'echo OK'
+    S:\> python3 run_quoted.py "'& echo Oh no!"
+    ''"'"'
+    Oh no!'
+
+Resolving this standard library limitation is beyond the scope of this PEP.
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.