Set weird behavior (python)

Zion Published at Dev

Zion

def _get_apps(path):
    """gets only all the apps"""

    return {app for app in os.listdir(path) if ".py" not in app}

apps = _get_apps(r"C:\Users\Hello\Desktop\Test")
css_apps = _get_apps(r"C:\Users\Hello\Desktop\Test2")

print(apps.difference(css_apps))

I'm trying to get the difference between two folders in my desktop. using the code above

individually the output is right it returns a set as expected

individual print:

print(apps)

print(css_apps)

Output:

{Music}

{Music,Css}

however doing:

print(apps.difference(css_apps))

Output:

set()

whats going on?

it returned a set as expected but somehow I can't do set operations on the returned set.

Rahul Gupta

This is because difference operation calculates the elements which are in apps set but not in css_apps set. Now there are no elements satisfying this criteria so you get an empty set.

s.difference(t) creates a :

new set with elements in s but not in t

Perhaps, what you need is .symmetric_difference(). This will create a new set with elements in either of the sets but not both.

In [1]: s1 = set([1]) # First set

In [2]: s2 = set([1,2]) # Second set

In [3]: s1.difference(s2) # Creates a new set with elements in s1 but not in s2
Out[3]: set() # empty because no elements satisfy this criteria

In [4]: s2.difference(s1)  # Creates a new set with elements in s2 but not in s1
Out[4]: {2} # element '2' exists in s2 but not in s1

In [5]: s1.symmetric_difference(s2) # Returns new set with elements in either s1 or s2 but not both
Out[5]: {2}

In [6]: s2.symmetric_difference(s1)
Out[6]: {2}

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