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Now type python and you should see it say python 2.7 blah blah blah; NOW for python 3.x. Simple, python 3.x download comes with python for windows app. SO simply pin the Python for Windows app to your task bar, or create shortcut to the desktop and you are done! Open Python for Windows for 3.x. Open Powershell for python 2.x. I hope this helps! Install OpenCV 4 in Python 3.7 / 2.7 30 December, 2019. OpenCV 4.2 works with Python 2.7, 3.5, 3.6, 3.7, 3.8. The unofficial OpenCV PyPi wheels work for Linux, Mac. Reading Time: 3 minutes The CentOS 7 Linux distribution includes Python 2 by default. However, Python 2 is going to reach EOL on January 1, 2020. While some legacy applications might require access to Python 2 for various reasons, it's vitally important to kick start new projects in Python 3.

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Many beginning Python users are wondering with which version of Python they should start. My answer to this question is usually something along the lines 'just go with the version your favorite tutorial was written in, and check out the differences later on.'

But what if you are starting a new project and have the choice to pick? I would say there is currently no 'right' or 'wrong' as long as both Python 2.7.x and Python 3.x support the libraries that you are planning to use. However, it is worthwhile to have a look at the major differences between those two most popular versions of Python to avoid common pitfalls when writing the code for either one of them, or if you are planning to port your project.

Sections

• The print function
• Integer division
• Unicode
• xrange
  • The __contains__ method for range objects in Python 3
• Raising exceptions
• Handling exceptions
• The next() function and .next() method
• For-loop variables and the global namespace leak
• Comparing unorderable types
• Parsing user inputs via input()
• Returning iterable objects instead of lists
• Banker's Rounding

The __future__ module

Python 3.x introduced some Python 2-incompatible keywords and features that can be imported via the in-built __future__ module in Python 2. It is recommended to use __future__ imports it if you are planning Python 3.x support for your code. For example, if we want Python 3.x's integer division behavior in Python 2, we can import it via

More features that can be imported from the __future__ module are listed in the table below:

feature	optional in	mandatory in	effect
nested_scopes	2.1.0b1	2.2	PEP 227:Statically Nested Scopes
generators	2.2.0a1	2.3	PEP 255:Simple Generators
division	2.2.0a2	3.0	PEP 238:Changing the Division Operator
absolute_import	2.5.0a1	3.0	PEP 328:Imports: Multi-Line and Absolute/Relative
with_statement	2.5.0a1	2.6	PEP 343:The 'with' Statement
print_function	2.6.0a2	3.0	PEP 3105:Make print a function
unicode_literals	2.6.0a2	3.0	PEP 3112:Bytes literals in Python 3000

(Source: [https://docs.python.org/2/library/__future__.html](https://docs.python.org/2/library/__future__.html#module-__future__))

The print function

Very trivial, and the change in the print-syntax is probably the most widely known change, but still it is worth mentioning: Python 2's print statement has been replaced by the print() function, meaning that we have to wrap the object that we want to print in parantheses.

Python 2 doesn't have a problem with additional parantheses, but in contrast, Python 3 would raise a SyntaxError if we called the print function the Python 2-way without the parentheses.

Python 2

Python 3

Note:

Printing 'Hello, World' above via Python 2 looked quite 'normal'. However, if we have multiple objects inside the parantheses, we will create a tuple, since print is a 'statement' in Python 2, not a function call.

Integer division

This change is particularly dangerous if you are porting code, or if you are executing Python 3 code in Python 2, since the change in integer-division behavior can often go unnoticed (it doesn't raise a SyntaxError).
So, I still tend to use a float(3)/2 or 3/2.0 instead of a 3/2 in my Python 3 scripts to save the Python 2 guys some trouble (and vice versa, I recommend a from __future__ import division in your Python 2 scripts).

Python 2

Python 3

Unicode

Python 2 has ASCII str() types, separate unicode(), but no byte type.

Now, in Python 3, we finally have Unicode (utf-8) strings, and 2 byte classes: byte and bytearrays.

Python 2

Python 3

xrange

The usage of xrange() is very popular in Python 2.x for creating an iterable object, e.g., in a for-loop or list/set-dictionary-comprehension.
The behavior was quite similar to a generator (i.e., 'lazy evaluation'), but here the xrange-iterable is not exhaustible - meaning, you could iterate over it infinitely.

Thanks to its 'lazy-evaluation', the advantage of the regular range() is that xrange() is generally faster if you have to iterate over it only once (e.g., in a for-loop). However, in contrast to 1-time iterations, it is not recommended if you repeat the iteration multiple times, since the generation happens every time from scratch!

In Python 3, the range() was implemented like the xrange() function so that a dedicated xrange() function does not exist anymore (xrange() raises a NameError in Python 3).

Python 2

Python 3

The __contains__ method for range objects in Python 3

Another thing worth mentioning is that range got a 'new' __contains__ method in Python 3.x (thanks to Yuchen Ying, who pointed this out). The __contains__ method can speedup 'look-ups' in Python 3.x range significantly for integer and Boolean types.

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Based on the timeit results above, you see that the execution for the 'look up' was about 60,000 faster when it was of an integer type rather than a float. However, since Python 2.x's range or xrange doesn't have a __contains__ method, the 'look-up speed' wouldn't be that much different for integers or floats:

Below the 'proofs' that the __contain__ method wasn't added to Python 2.x yet:

Note about the speed differences in Python 2 and 3

Some people pointed out the speed difference between Python 3's range() and Python2's xrange(). Since they are implemented the same way one would expect the same speed. However the difference here just comes from the fact that Python 3 generally tends to run slower than Python 2.

Raising exceptions

Where Python 2 accepts both notations, the ‘old' and the ‘new' syntax, Python 3 chokes (and raises a SyntaxError in turn) if we don't enclose the exception argument in parentheses:

Python 2

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Python 3

The proper way to raise an exception in Python 3:

Handling exceptions

Also the handling of exceptions has slightly changed in Python 3. In Python 3 we have to use the 'as' keyword now

Python 2

Python 3

The next() function and .next() method

Since next() (.next()) is such a commonly used function (method), this is another syntax change (or rather change in implementation) that is worth mentioning: where you can use both the function and method syntax in Python 2.7.5, the next() function is all that remains in Python 3 (calling the .next() method raises an AttributeError).

Python 2

Python 3

For-loop variables and the global namespace leak

Good news is: In Python 3.x for-loop variables don't leak into the global namespace anymore!

This goes back to a change that was made in Python 3.x and is described in What's New In Python 3.0 as follows:

'List comprehensions no longer support the syntactic form [.. for var in item1, item2, ..]. Use [.. for var in (item1, item2, ..)] instead. Also note that list comprehensions have different semantics: they are closer to syntactic sugar for a generator expression inside a list() constructor, and in particular the loop control variables are no longer leaked into the surrounding scope.'

Python 2

Python 3

Comparing unorderable types

Another nice change in Python 3 is that a TypeError is raised as warning if we try to compare unorderable types.

Python 2

Python 3

Parsing user inputs via input()

Fortunately, the input() function was fixed in Python 3 so that it always stores the user inputs as str objects. In order to avoid the dangerous behavior in Python 2 to read in other types than strings, we have to use raw_input() instead.

Python 2

Python 3

Returning iterable objects instead of lists

As we have already seen in the xrange section, some functions and methods return iterable objects in Python 3 now - instead of lists in Python 2.

Since we usually iterate over those only once anyway, I think this change makes a lot of sense to save memory. However, it is also possible - in contrast to generators - to iterate over those multiple times if needed, it is only not so efficient.

And for those cases where we really need the list-objects, we can simply convert the iterable object into a list via the list() function.

Python 2

Python 3

Some more commonly used functions and methods that don't return lists anymore in Python 3:

• zip()

• map()

• filter()

• dictionary's .keys() method

• dictionary's .values() method

• dictionary's .items() method

Banker's Rounding

Python 3 adopted the now standard way of rounding decimals when it results in a tie (.5) at the last significant digits. Now, in Python 3, decimals are rounded to the nearest even number. Although it's an inconvenience for code portability, it's supposedly a better way of rounding compared to rounding up as it avoids the bias towards large numbers. For more information, see the excellent Wikipedia articles and paragraphs:

Python 2

Python 3

More articles about Python 2 and Python 3

Here is a list of some good articles concerning Python 2 and 3 that I would recommend as a follow-up.

// Porting to Python 3

// Pro and anti Python 3

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