How do I check which version of Python 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.

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 :)

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")

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

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.

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.

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)