Python 3: checking version before syntactic analysis

Question

I would like to check if my interpreter is using Python 3, before parsing the entire file, to print a decent error message otherwise.

In this question, there is an answer regarding Python 2.4 / 2.5. However, when I tried to reproduce the solution in my Python 2.7 / 3.3 environment, it did not work.

For instance, running the following file:

try:
  eval("print('', end='')")
except SyntaxError:
  raise Exception("python 3 needed")

print("", end="")

Results in the following error:

File "test_version.py", line 6
  print("", end="")
             ^
SyntaxError: invalid syntax

That is, eval is not evaluated soon enough to prevent the parser from reaching line 6.

Is there another hack that might work here?

The code posted here would not work on Python 2.4 or 2.5 either; you are using `print()` as a function in the code, so the compiler will throw the exception anyway. Move the `print()` use *out* of this module into a separate module. — Martijn Pieters, Feb 10 '14 at 10:32

Apalala · Answer 1 · 2014-02-10T17:06:45.220

2

The classic way to check for the version of Python that is running your script is to use the tuple returned by sys.version_info (http://docs.python.org/2/library/sys.html):

from __future__ import print_function
import sys
if sys.version_info[0] < 3:
    print("This program needs Python version 3.2 or later")
    sys.exit(1)

The reason to enable print() as a function through the import from __future__ is because the script wouldn't parse under Python 3.x if the print statement was used.

edited Feb 10 '14 at 17:06

answered Feb 10 '14 at 16:38

Apalala

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Could you explain your answer? – everton Feb 10 '14 at 17:00
@EvertonAgner Using `sys.version_info` will give you information about the version of Python you are using. This is how you can determine if you are using a version you need - just as a general tip. – 2rs2ts Feb 10 '14 at 17:19

score 1 · Accepted Answer · answered Feb 10 '14 at 10:33

Move code using print() as a function to a separate module and make the main script do the testing only.

It doesn't matter where in the module the print() function call is made, it is a syntax error because you are not using print as a statement. You'd put that in a separate module imported after your test.

try:
    eval("print('', end='')")
except SyntaxError:
    raise Exception("python 3 needed")

import real_script_using_print_function

However, in Python 2 you can also add:

from __future__ import print_function

at the top to make print() work in Python 2 as well. It is perfectly feasible to write Python code that runs on both Python 2 and 3 if taken care.

The `from __future__ import print_function` is exactly what I needed. I cannot add another "main" file to replace the current one, but I can add the import and then test the version without errors. — DanGar, Feb 10 '14 at 11:08

Python 3: checking version before syntactic analysis

2 Answers2