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How does the Python interpreter know what distribution it is in? [duplicate]

How do I check which version of the Python interpreter is running my script?

This information is available in the sys.version string in the sys module:
>>> import sys
Human readable:
>>> print(sys.version) # parentheses necessary in python 3.
2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)]
For further processing, use sys.version_info or sys.hexversion:
>>> sys.version_info
(2, 5, 2, 'final', 0)
# or
>>> sys.hexversion
34014192
To ensure a script runs with a minimal version requirement of the Python interpreter add this to your code:
assert sys.version_info >= (2, 5)
This compares major and minor version information. Add micro (=0, 1, etc) and even releaselevel (='alpha','final', etc) to the tuple as you like. Note however, that it is almost always better to "duck" check if a certain feature is there, and if not, workaround (or bail out). Sometimes features go away in newer releases, being replaced by others.

From the command line (note the capital 'V'):
python -V
This is documented in 'man python'.
From IPython console
!python -V

Use platform's python_version from the stdlib:
from platform import python_version
print(python_version())
# 3.9.2

I like sys.hexversion for stuff like this.
>>> import sys
>>> sys.hexversion
33883376
>>> '%x' % sys.hexversion
'20504f0'
>>> sys.hexversion < 0x02060000
True

Your best bet is probably something like so:
>>> import sys
>>> sys.version_info
(2, 6, 4, 'final', 0)
>>> if not sys.version_info[:2] == (2, 6):
... print "Error, I need python 2.6"
... else:
... from my_module import twoPointSixCode
>>>
Additionally, you can always wrap your imports in a simple try, which should catch syntax errors. And, to #Heikki's point, this code will be compatible with much older versions of python:
>>> try:
... from my_module import twoPointSixCode
... except Exception:
... print "can't import, probably because your python is too old!"
>>>

Put something like:
#!/usr/bin/env/python
import sys
if sys.version_info<(2,6,0):
sys.stderr.write("You need python 2.6 or later to run this script\n")
exit(1)
at the top of your script.
Note that depending on what else is in your script, older versions of python than the target may not be able to even load the script, so won't get far enough to report this error. As a workaround, you can run the above in a script that imports the script with the more modern code.

Here's a short commandline version which exits straight away (handy for scripts and automated execution):
python -c "print(__import__('sys').version)"
Or just the major, minor and micro:
python -c "print(__import__('sys').version_info[:1])" # (2,)
python -c "print(__import__('sys').version_info[:2])" # (2, 7)
python -c "print(__import__('sys').version_info[:3])" # (2, 7, 6)

With six module, you can do it by:
import six
if six.PY2:
# this is python2.x
else:
# six.PY3
# this is python3.x

import sys
sys.version.split(' ')[0]
sys.version gives you what you want, just pick the first number :)

Like Seth said, the main script could check sys.version_info (but note that that didn't appear until 2.0, so if you want to support older versions you would need to check another version property of the sys module).
But you still need to take care of not using any Python language features in the file that are not available in older Python versions. For example, this is allowed in Python 2.5 and later:
try:
pass
except:
pass
finally:
pass
but won't work in older Python versions, because you could only have except OR finally match the try. So for compatibility with older Python versions you need to write:
try:
try:
pass
except:
pass
finally:
pass

Several answers already suggest how to query the current python version. To check programmatically the version requirements, I'd make use of one of the following two methods:
# Method 1: (see krawyoti's answer)
import sys
assert(sys.version_info >= (2,6))
# Method 2:
import platform
from distutils.version import StrictVersion
assert(StrictVersion(platform.python_version()) >= "2.6")

Just for fun, the following is a way of doing it on CPython 1.0-3.7b2, Pypy, Jython and Micropython. This is more of a curiosity than a way of doing it in modern code. I wrote it as part of http://stromberg.dnsalias.org/~strombrg/pythons/ , which is a script for testing a snippet of code on many versions of python at once, so you can easily get a feel for what python features are compatible with what versions of python:
via_platform = 0
check_sys = 0
via_sys_version_info = 0
via_sys_version = 0
test_sys = 0
try:
import platform
except (ImportError, NameError):
# We have no platform module - try to get the info via the sys module
check_sys = 1
if not check_sys:
if hasattr(platform, "python_version"):
via_platform = 1
else:
check_sys = 1
if check_sys:
try:
import sys
test_sys = 1
except (ImportError, NameError):
# just let via_sys_version_info and via_sys_version remain False - we have no sys module
pass
if test_sys:
if hasattr(sys, "version_info"):
via_sys_version_info = 1
elif hasattr(sys, "version"):
via_sys_version = 1
else:
# just let via_sys remain False
pass
if via_platform:
# This gives pretty good info, but is not available in older interpreters. Also, micropython has a
# platform module that does not really contain anything.
print(platform.python_version())
elif via_sys_version_info:
# This is compatible with some older interpreters, but does not give quite as much info.
print("%s.%s.%s" % sys.version_info[:3])
elif via_sys_version:
import string
# This is compatible with some older interpreters, but does not give quite as much info.
verbose_version = sys.version
version_list = string.split(verbose_version)
print(version_list[0])
else:
print("unknown")

If you want to detect pre-Python 3 and don't want to import anything...
...you can (ab)use list comprehension scoping changes and do it in a single expression:
is_python_3_or_above = (lambda x: [x for x in [False]] and None or x)(True)

from sys import version_info, api_version, version, hexversion
print(f"sys.version: {version}")
print(f"sys.api_version: {api_version}")
print(f"sys.version_info: {version_info}")
print(f"sys.hexversion: {hexversion}")
output
sys.version: 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 17:00:18) [MSC v.1900 64 bit (AMD64)]
sys.api_version: 1013
sys.version_info: sys.version_info(major=3, minor=6, micro=5, releaselevel='final', serial=0)
sys.hexversion: 50726384

The simplest way
Just type python in your terminal and you can see the version
as like following
desktop:~$ python
Python 2.7.6 (default, Jun 22 2015, 18:00:18)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

sys.version_info doesn't seem to return a tuple as of 3.7. Rather, it returns a special class, so all of the examples using tuples don't work, for me at least. Here's the output from a python console:
>>> import sys
>>> type(sys.version_info)
<class 'sys.version_info'>
I've found that using a combination of sys.version_info.major and sys.version_info.minor seems to suffice. For example,...
import sys
if sys.version_info.major > 3:
print('Upgrade to Python 3')
exit(1)
checks if you're running Python 3. You can even check for more specific versions with...
import sys
ver = sys.version_info
if ver.major > 2:
if ver.major == 3 and ver.minor <= 4:
print('Upgrade to Python 3.5')
exit(1)
can check to see if you're running at least Python 3.5.

Check Python version: python -V or python --version or apt-cache policy python
you can also run whereis python to see how many versions are installed.

To verify the Python version for commands on Windows, run the following commands in a command prompt and verify the output
c:\>python -V
Python 2.7.16
c:\>py -2 -V
Python 2.7.16
c:\>py -3 -V
Python 3.7.3
Also, To see the folder configuration for each Python version, run the following commands:
For Python 2,'py -2 -m site'
For Python 3,'py -3 -m site'

This just returns 2.7, 3.6 or 3.9
import sys
current_version = ".".join(map(str, sys.version_info[0:2]))
which is what you usually you need...

The even simpler simplest way:
In Spyder, start a new "IPython Console", then run any of your existing scripts.
Now the version can be seen in the first output printed in the console window:
"Python 3.7.3 (default, Apr 24 2019, 15:29:51)..."

To check from the command-line, in one single command, but include major, minor, micro version, releaselevel and serial, then invoke the same Python interpreter (i.e. same path) as you're using for your script:
> path/to/your/python -c "import sys; print('{}.{}.{}-{}-{}'.format(*sys.version_info))"
3.7.6-final-0
Note: .format() instead of f-strings or '.'.join() allows you to use arbitrary formatting and separator chars, e.g. to make this a greppable one-word string. I put this inside a bash utility script that reports all important versions: python, numpy, pandas, sklearn, MacOS, xcode, clang, brew, conda, anaconda, gcc/g++ etc. Useful for logging, replicability, troubleshootingm bug-reporting etc.

all answers has great insights
another way is platform.python_version_tuple()
python3 -c “import platform; print(platform.python_version_tuple())”
(‘3’, ‘6’, ‘8’)

if you want to check the python version for at least condition (e.g., python 3.9.0):
import platform
python_version_tuple = list(map(int, platform.python_version_tuple()))
if python_version_tuple >= [3, 9, 0]:
# Running at least Python 3.9.0
else:
# Running Python below 3.9.0

If you are working on linux just give command python output will be like this
Python 2.4.3 (#1, Jun 11 2009, 14:09:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-44)] on linux2
Type "help", "copyright", "credits" or "license" for more
information.

For windows, Go to command prompt and type this command to get the python version:
python --version
Or
python -V

A attempt using os.popen to read it in a variable:
import os
ver = os.popen('python -V').read().strip()
print(ver)

Can't popen new instance of python.exe

There’s a project where I need to use both Python 3.3 and 2.7. I am trying to launch a script under Python 2.7 but it’s not working. Here is a simple example.
first.py
import subprocess
import sys
print('Inside first.py')
print(sys.version)
subprocess.Popen(["C:\Python27\ArcGISx6410.2\Python.exe", "second.py"])
second.py
import arcpy
print 'This is second.py'
This doesn’t work and the output is
Inside first.py
3.3.5 (v3.3.5:62cf4e77f785, Mar 9 2014, 10:35:05) [MSC v.1600 64 bit (AMD64)]
File "C:\Python33\lib\site.py", line 173
file=sys.stderr)
^
SyntaxError: invalid syntax
That’s the entire stack trace. If I were to replace C:\...Python.exe with notepad.exe then it works. I’m using Liclipse on Windows 7.
UPDATE: it appears different versions of Python are run, when from the command line python first.py is 3.3 but py first.py or just first.py then 2.7 is used.

Try:
import os
subprocess.Popen(["C:\\Python27\\ArcGISx6410.2\\Python.exe", "second.py"], env=dict(os.environ, PYTHONHOME="C:\\Python27\\ArcGISx6410.2"))
Python on Windows needs a little help sometimes to figure out which version of the standard library to use.

python idle 2.7.9 gives a false syntax error

I have a file: pytest.py which is one line:
print "hello world"
idle -r pytest.py
Python 2.7.9 (default, Apr 8 2015, 15:12:41)
[GCC 4.2.1 Compatible FreeBSD Clang 3.4.1 (tags/RELEASE_34/dot1-final 208032)] on freebsd10
Type "copyright", "credits" or "license()" for more information.
>>>
*** Error in script or command!
Traceback (most recent call last):
File "pytest.py", line 1
print "hello world"
^
SyntaxError: invalid syntax
>>>
running idle with no options, opening and running the file works. This is on FreeBSD 10.0 and py27-gtk2-2.24.0_3 (python binding). This stopped working at some point but I can not relate it to a specific change. All packages/port are up-to-date

Idle compiles the -r code as follows:
code = compile(source, filename, "exec")
However, by default, compile inherits the calling code's future settings:
The optional arguments flags and dont_inherit control which future statements (see PEP 236) affect the compilation of source. If neither is present (or both are zero) the code is compiled with those future statements that are in effect in the code that is calling compile()
Since idle's PyShell.py module does enable the print_function future flags, this means that by accident all of your code in -r has to use it to.
Change your code to print("Hello world") to fix the problem. As a nice side effect, your code will work in Python 3.x as well.

I caused this problem when I added
from __future__ import print_function
to the top of PyShell.py as part of backporting the bugfix in Issue 22420. The fix, which I just applied in Issue 24222, is this change to line 655.
- code = compile(source, filename, "exec")
+ code = compile(source, filename, "exec", dont_inherit=True)
Thanks to 'phihag' for pointing out the problem line.

from future import print_function
print "hello world"
gives the same result on my system
Oh poo can't use vi -- the answer was correct

Gl3w Python script errors: how to solve it/get it working

So after almost 2 days of not being able to get GLEW working: trying Gl3w.
The python script is complaining about Syntax Errors, said all the prints signaled a syntax error, so I removed them all. Now it's complaining about the imports... (basically it seems it's complaining of the entire code)
I don't know Python, but I can't find support for Gl3w... should I keep trying to "correct the script", which I think should be working fine? What am I "doing wrong"?
Thanks!
This is the script: https://github.com/skaslev/gl3w

You are using Python 3, while the script requires Python 2:
Its main part is a simple gl3w_gen.py Python 2.6 script
In python 2, print is a statement, in 3, it's a function. This difference leads to syntax errors when trying to run python 2 code with python 3:
$ python3.3
Python 3.3.0 (default, Sep 29 2012, 08:16:08)
[GCC 4.2.1 Compatible Apple Clang 3.1 (tags/Apple/clang-318.0.58)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print 'Hello world!'
File "<stdin>", line 1
print 'Hello world!'
^
SyntaxError: invalid syntax

Determing Python version at runtime [duplicate]

How do I check which version of the Python interpreter is running my script?

This information is available in the sys.version string in the sys module:
>>> import sys
Human readable:
>>> print(sys.version) # parentheses necessary in python 3.
2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)]
For further processing, use sys.version_info or sys.hexversion:
>>> sys.version_info
(2, 5, 2, 'final', 0)
# or
>>> sys.hexversion
34014192
To ensure a script runs with a minimal version requirement of the Python interpreter add this to your code:
assert sys.version_info >= (2, 5)
This compares major and minor version information. Add micro (=0, 1, etc) and even releaselevel (='alpha','final', etc) to the tuple as you like. Note however, that it is almost always better to "duck" check if a certain feature is there, and if not, workaround (or bail out). Sometimes features go away in newer releases, being replaced by others.

From the command line (note the capital 'V'):
python -V
This is documented in 'man python'.
From IPython console
!python -V

Use platform's python_version from the stdlib:
from platform import python_version
print(python_version())
# 3.9.2

I like sys.hexversion for stuff like this.
>>> import sys
>>> sys.hexversion
33883376
>>> '%x' % sys.hexversion
'20504f0'
>>> sys.hexversion < 0x02060000
True

Your best bet is probably something like so:
>>> import sys
>>> sys.version_info
(2, 6, 4, 'final', 0)
>>> if not sys.version_info[:2] == (2, 6):
... print "Error, I need python 2.6"
... else:
... from my_module import twoPointSixCode
>>>
Additionally, you can always wrap your imports in a simple try, which should catch syntax errors. And, to #Heikki's point, this code will be compatible with much older versions of python:
>>> try:
... from my_module import twoPointSixCode
... except Exception:
... print "can't import, probably because your python is too old!"
>>>

Put something like:
#!/usr/bin/env/python
import sys
if sys.version_info<(2,6,0):
sys.stderr.write("You need python 2.6 or later to run this script\n")
exit(1)
at the top of your script.
Note that depending on what else is in your script, older versions of python than the target may not be able to even load the script, so won't get far enough to report this error. As a workaround, you can run the above in a script that imports the script with the more modern code.

Here's a short commandline version which exits straight away (handy for scripts and automated execution):
python -c "print(__import__('sys').version)"
Or just the major, minor and micro:
python -c "print(__import__('sys').version_info[:1])" # (2,)
python -c "print(__import__('sys').version_info[:2])" # (2, 7)
python -c "print(__import__('sys').version_info[:3])" # (2, 7, 6)

With six module, you can do it by:
import six
if six.PY2:
# this is python2.x
else:
# six.PY3
# this is python3.x

import sys
sys.version.split(' ')[0]
sys.version gives you what you want, just pick the first number :)

Like Seth said, the main script could check sys.version_info (but note that that didn't appear until 2.0, so if you want to support older versions you would need to check another version property of the sys module).
But you still need to take care of not using any Python language features in the file that are not available in older Python versions. For example, this is allowed in Python 2.5 and later:
try:
pass
except:
pass
finally:
pass
but won't work in older Python versions, because you could only have except OR finally match the try. So for compatibility with older Python versions you need to write:
try:
try:
pass
except:
pass
finally:
pass

Several answers already suggest how to query the current python version. To check programmatically the version requirements, I'd make use of one of the following two methods:
# Method 1: (see krawyoti's answer)
import sys
assert(sys.version_info >= (2,6))
# Method 2:
import platform
from distutils.version import StrictVersion
assert(StrictVersion(platform.python_version()) >= "2.6")

Just for fun, the following is a way of doing it on CPython 1.0-3.7b2, Pypy, Jython and Micropython. This is more of a curiosity than a way of doing it in modern code. I wrote it as part of http://stromberg.dnsalias.org/~strombrg/pythons/ , which is a script for testing a snippet of code on many versions of python at once, so you can easily get a feel for what python features are compatible with what versions of python:
via_platform = 0
check_sys = 0
via_sys_version_info = 0
via_sys_version = 0
test_sys = 0
try:
import platform
except (ImportError, NameError):
# We have no platform module - try to get the info via the sys module
check_sys = 1
if not check_sys:
if hasattr(platform, "python_version"):
via_platform = 1
else:
check_sys = 1
if check_sys:
try:
import sys
test_sys = 1
except (ImportError, NameError):
# just let via_sys_version_info and via_sys_version remain False - we have no sys module
pass
if test_sys:
if hasattr(sys, "version_info"):
via_sys_version_info = 1
elif hasattr(sys, "version"):
via_sys_version = 1
else:
# just let via_sys remain False
pass
if via_platform:
# This gives pretty good info, but is not available in older interpreters. Also, micropython has a
# platform module that does not really contain anything.
print(platform.python_version())
elif via_sys_version_info:
# This is compatible with some older interpreters, but does not give quite as much info.
print("%s.%s.%s" % sys.version_info[:3])
elif via_sys_version:
import string
# This is compatible with some older interpreters, but does not give quite as much info.
verbose_version = sys.version
version_list = string.split(verbose_version)
print(version_list[0])
else:
print("unknown")

If you want to detect pre-Python 3 and don't want to import anything...
...you can (ab)use list comprehension scoping changes and do it in a single expression:
is_python_3_or_above = (lambda x: [x for x in [False]] and None or x)(True)

from sys import version_info, api_version, version, hexversion
print(f"sys.version: {version}")
print(f"sys.api_version: {api_version}")
print(f"sys.version_info: {version_info}")
print(f"sys.hexversion: {hexversion}")
output
sys.version: 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 17:00:18) [MSC v.1900 64 bit (AMD64)]
sys.api_version: 1013
sys.version_info: sys.version_info(major=3, minor=6, micro=5, releaselevel='final', serial=0)
sys.hexversion: 50726384

The simplest way
Just type python in your terminal and you can see the version
as like following
desktop:~$ python
Python 2.7.6 (default, Jun 22 2015, 18:00:18)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

sys.version_info doesn't seem to return a tuple as of 3.7. Rather, it returns a special class, so all of the examples using tuples don't work, for me at least. Here's the output from a python console:
>>> import sys
>>> type(sys.version_info)
<class 'sys.version_info'>
I've found that using a combination of sys.version_info.major and sys.version_info.minor seems to suffice. For example,...
import sys
if sys.version_info.major > 3:
print('Upgrade to Python 3')
exit(1)
checks if you're running Python 3. You can even check for more specific versions with...
import sys
ver = sys.version_info
if ver.major > 2:
if ver.major == 3 and ver.minor <= 4:
print('Upgrade to Python 3.5')
exit(1)
can check to see if you're running at least Python 3.5.

Check Python version: python -V or python --version or apt-cache policy python
you can also run whereis python to see how many versions are installed.

To verify the Python version for commands on Windows, run the following commands in a command prompt and verify the output
c:\>python -V
Python 2.7.16
c:\>py -2 -V
Python 2.7.16
c:\>py -3 -V
Python 3.7.3
Also, To see the folder configuration for each Python version, run the following commands:
For Python 2,'py -2 -m site'
For Python 3,'py -3 -m site'

This just returns 2.7, 3.6 or 3.9
import sys
current_version = ".".join(map(str, sys.version_info[0:2]))
which is what you usually you need...

The even simpler simplest way:
In Spyder, start a new "IPython Console", then run any of your existing scripts.
Now the version can be seen in the first output printed in the console window:
"Python 3.7.3 (default, Apr 24 2019, 15:29:51)..."

To check from the command-line, in one single command, but include major, minor, micro version, releaselevel and serial, then invoke the same Python interpreter (i.e. same path) as you're using for your script:
> path/to/your/python -c "import sys; print('{}.{}.{}-{}-{}'.format(*sys.version_info))"
3.7.6-final-0
Note: .format() instead of f-strings or '.'.join() allows you to use arbitrary formatting and separator chars, e.g. to make this a greppable one-word string. I put this inside a bash utility script that reports all important versions: python, numpy, pandas, sklearn, MacOS, xcode, clang, brew, conda, anaconda, gcc/g++ etc. Useful for logging, replicability, troubleshootingm bug-reporting etc.

all answers has great insights
another way is platform.python_version_tuple()
python3 -c “import platform; print(platform.python_version_tuple())”
(‘3’, ‘6’, ‘8’)

if you want to check the python version for at least condition (e.g., python 3.9.0):
import platform
python_version_tuple = list(map(int, platform.python_version_tuple()))
if python_version_tuple >= [3, 9, 0]:
# Running at least Python 3.9.0
else:
# Running Python below 3.9.0

If you are working on linux just give command python output will be like this
Python 2.4.3 (#1, Jun 11 2009, 14:09:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-44)] on linux2
Type "help", "copyright", "credits" or "license" for more
information.

For windows, Go to command prompt and type this command to get the python version:
python --version
Or
python -V

A attempt using os.popen to read it in a variable:
import os
ver = os.popen('python -V').read().strip()
print(ver)