peps: 4b379a690ae2 (original) (raw)

Mercurial > peps

changeset 5007:4b379a690ae2

Update PEP 394 based on python-dev discussions

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--- a/pep-0394.txt +++ b/pep-0394.txt @@ -19,10 +19,17 @@ This PEP provides a convention to ensure be portable across *nix systems, regardless of the default version of the Python interpreter (i.e. the version invoked by the python command). -* python2 will refer to some version of Python 2.x -* python3 will refer to some version of Python 3.x -* python should refer to the same target as python2 but may

• refer to python3 on some bleeding edge distributions +* python2 will refer to some version of Python 2.x. +* python3 will refer to some version of Python 3.x. +* for the time being, all distributions should ensure that python

• refers to the same target as python2. +* however, end users should be aware that python refers to python3
• on at least Arch Linux (that change is what prompted the creation of this
• PEP), so python should be used in the shebang line only for scripts
• that are source compatible with both Python 2 and 3. +* in preparation for an eventual change in the default version of Python,
• Python 2 only scripts should either be updated to be source compatible
• with Python 3 or else to use python2 in the shebang line. Recommendation @@ -103,15 +110,29 @@ aspects of migrating to Python 3 as the system. They will hopefully be helpful to any distributions considering making such a change. -* Distributions that only include python3 in their base install (i.e.

• they do not provide python2 by default) along with those that are
• aggressively trying to reach that point (and are willing to break third
• party scripts while attempting to get there) are already beginning to alias
• the python command to python3 -* More conservative distributions that are less willing to tolerate breakage
• of third party scripts continue to alias it to python2. Until the
• conventions described in this PEP are more widely adopted, having python
• invoke python2 will remain the recommended option. +* The main barrier to a distribution switching the python command from

• python2 to python3 isn't breakage within the distribution, but
• instead breakage of private third party scripts developed by sysadmins
• and other users. Updating the python command to invoke python3
• by default indicates that a distribution is willing to break such scripts
• with errors that are potentially quite confusing for users that aren't
• yet familiar with the backwards incompatible changes in Python 3. For
• example, while the change of print from a statement to a builtin
• function is relatively simple for automated converters to handle, the
• SyntaxError from attempting to use the Python 2 notation in Python 3 is
• thoroughly confusing if you aren't already aware of the change:: +

•  $ python3 -c 'print "Hello, world!"'[](#l1.50)

•    File "<string>", line 1[](#l1.51)

•      print "Hello, world!"[](#l1.52)

•                          ^[](#l1.53)

•  SyntaxError: invalid syntax[](#l1.54)

+ +* Avoiding breakage of such third party scripts is the key reason this

• PEP recommends that python continue to refer to python2 for the
• time being. Until the conventions described in this PEP are more widely
• adopted, having python invoke python2 will remain the recommended
• option.

• The pythonX.X (e.g. python2.6) commands exist on some systems, on which they invoke specific minor versions of the Python interpreter. It can be useful for distribution-specific packages to take advantage of these @@ -148,10 +169,13 @@ making such a change. python command is only executed in an interactive manner as a user convenience, or to run scripts that are source compatible with both Python 2 and Python 3. +* one symbolic date being considered for a possible change to the official

• recommendation in this PEP is the planned switch of Python 2.7 from full
• maintenance to security update only status in 2015 (see PEP 373). Backwards Compatibility -========================= +======================= A potential problem can arise if a script adhering to the python2/python3 convention is executed on a system not supporting @@ -217,7 +241,8 @@ Exclusion of MS Windows This PEP deliberately excludes any proposals relating to Microsoft Windows, as devising an equivalent solution for Windows was deemed too complex to handle here. PEP 397 and the related discussion on the python-dev mailing list -address this issue. +address this issue (like this PEP, the PEP 397 launcher invokes Python 2 by +default if versions of both Python 2 and 3 are installed on the system). References