Asked  7 Months ago    Answers:  5   Viewed   25 times

I don't understand the following from pep-0404

In Python 3, implicit relative imports within packages are no longer available - only absolute imports and explicit relative imports are supported. In addition, star imports (e.g. from x import *) are only permitted in module level code.

What is a relative import? In what other places star import was allowed in python2? Please explain with examples.

 Answers

22

Relative import happens whenever you are importing a package relative to the current script/package.

Consider the following tree for example:

mypkg
??? base.py
??? derived.py

Now, your derived.py requires something from base.py. In Python 2, you could do it like this (in derived.py):

from base import BaseThing

Python 3 no longer supports that since it's not explicit whether you want the 'relative' or 'absolute' base. In other words, if there was a Python package named base installed in the system, you'd get the wrong one.

Instead it requires you to use explicit imports which explicitly specify location of a module on a path-alike basis. Your derived.py would look like:

from .base import BaseThing

The leading . says 'import base from module directory'; in other words, .base maps to ./base.py.

Similarly, there is .. prefix which goes up the directory hierarchy like ../ (with ..mod mapping to ../mod.py), and then ... which goes two levels up (../../mod.py) and so on.

Please however note that the relative paths listed above were relative to directory where current module (derived.py) resides in, not the current working directory.


@BrenBarn has already explained the star import case. For completeness, I will have to say the same ;).

For example, you need to use a few math functions but you use them only in a single function. In Python 2 you were permitted to be semi-lazy:

def sin_degrees(x):
    from math import *
    return sin(degrees(x))

Note that it already triggers a warning in Python 2:

a.py:1: SyntaxWarning: import * only allowed at module level
  def sin_degrees(x):

In modern Python 2 code you should and in Python 3 you have to do either:

def sin_degrees(x):
    from math import sin, degrees
    return sin(degrees(x))

or:

from math import *

def sin_degrees(x):
    return sin(degrees(x))
Tuesday, June 1, 2021
 
mschuett
answered 7 Months ago
60

You are running foo.py like a script, but you are really using it like a module. So the proper solution is to run it as a module:

python3 -m somepackage.foo

For the record, another alternative is to edit your path like:

export PYTHONPATH=.

(Or you could put the absolute directory in there, and of course you should append any other directories that are already in your PYTHONPATH.) This is closer to what PyCharm does, but is less philosophically correct.

Thursday, July 1, 2021
 
mnagel
answered 6 Months ago
98

It's the most common style to put every import at the top of the file. PEP 8 recommends it, which is a good reason to do it to start with. But that's not a whim, it has advantages (although not critical enough to make everything else a crime). It allows finding all imports at a glance, as opposed to looking through the whole file. It also ensures everything is imported before any other code (which may depend on some imports) is executed. NameErrors are usually easy to resolve, but they can be annoying.

There's no (significant) namespace pollution to be avoided by keeping the module in a smaller scope, since all you add is the actual module (no, import * doesn't count and probably shouldn't be used anyway). Inside functions, you'd import again on every call (not really harmful since everything is imported once, but uncalled for).

Saturday, July 17, 2021
 
SpiderLinked
answered 5 Months ago
75

The reload built-in function has been moved to importlib module in Python 3.4:

In [18]: from importlib import reload

In [19]: reload?
Reload the module and return it.

The module must have been successfully imported before.

As pointed out by @JPaget in comments reload() function has been moved from imp to importlib module in Python 3.4+. From what's new in Python 3.4:

The reload() function has been moved from imp to importlib as part of the imp module deprecation

Thursday, August 12, 2021
 
bimal
answered 4 Months ago
58

use the following to convert to a timestamp in python 2

int((mod_time.mktime(first_run.timetuple())+first_run.microsecond/1000000.0))

Sunday, August 22, 2021
 
waylaidwanderer
answered 4 Months ago
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