How does value asignments within Tuples in Python work?

Question

While working on a script in Python, I came accross this built-in method part of the sys module sys.flags which returns a tuple of flags used to run the python script. The output looks like:

(debug=0, inspect=0, interactive=0, optimize=1, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=0, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=0, dev_mode=False, utf8_mode=0)

I'm confused, as I thought you could not embed an assignment within another expression in Python, and so far I haven't been able to find an answer on Google that explains the behavior of this tuple.

`sys.flags` is not a tuple; `type(sys.flags)` reveals that much. — kaya3, Dec 10 '19 at 20:03
I think those are the current settings as opposed to assignments. There might be a `__str__` method that prints the output you are seeing as a reference, similar to what is discussed here https://stackoverflow.com/questions/1535327/how-to-print-instances-of-a-class-using-print — chrisbyte, Dec 10 '19 at 20:04

ForceBru · Answer 1 · 2019-12-10T20:11:59.063

The actual output is:

>>> sys.flags
sys.flags(debug=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=0, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=0, dev_mode=False, utf8_mode=0)

That sys.flags before the opening parenthesis is crucial. It shows that sys.flags is something else than a tuple.

Yet you could totally create a class whose __repr__ method would return this kind of tuple-like representation:

class Weird:
    def __init__(self, a, b):
        self.a, self.b = a, b

    def __repr__(self):
        return f"(a={self.a}, b={self.b})"

Then the output will look like this:

>>> Weird(1,2)
(a=1, b=2)

But Weird is definitely not a tuple.

MSeifert · Accepted Answer · 2019-12-10T20:18:44.253

The output doesn't show a tuple:

>>> import sys
>>> sys.flags
sys.flags(debug=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=0, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=0, dev_mode=False, utf8_mode=0)
>>> type(sys.flags)
<class 'sys.flags'>

A tuple would look like this:

>>> tuple(sys.flags)
(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, False, 0)

It actually returns an instance of a class that has a custom representation.

Note that this is a typical representation for a "data-class". It shows how such an object was created or could be created. For example namedtuple have a similar representation (not an accident because sys.flags is a namedtuple!):

>>> from collections import namedtuple
>>> Person = namedtuple('Person', 'name, age')
>>> Person(20, 'me')
Person(name=20, age='me')
>>> Person(name=20, age='me')
Person(name=20, age='me')

score 2 · Answer 3 · answered Dec 10 '19 at 20:08

Indeed, doing:

isinstance(sys.flags, tuple)

will return True. But that is because sys.flags is a named-tuple, which inherits from the tuple type. From the docs:

The term “named tuple” applies to any type or class that inherits from tuple and whose indexable elements are also accessible using named attributes.

How does value asignments within Tuples in Python work?

3 Answers3