Python’s `set.pop()`

method removes and returns an arbitrary element from the set. If no element is present — that is, you call it on an empty set — `set.pop()`

raises a `KeyError`

.

Here’s a minimal example where you remove a string element that happens to be `'Alice'`

from the set by means of the `s.pop()`

method:

>>> s = {'Alice', 'Bob', 'Carl'} >>> s.pop() 'Alice'

Table of Contents

## Syntax

Let’s dive into the formal syntax of the `set.pop()`

method.

set.pop()

Argument | Data Type | Explanation |
---|---|---|

`-` | – | – |

## Python Set pop() a Specific Element

? The `set.pop()`

method doesn’t take an argument. If you still pass an argument, it raises a `TypeError: pop() takes no arguments (1 given)`

. To fix this, don’t pass an argument into the method. If you need to remove a specific element from the set, use the `set.remove(x)`

method, or, to avoid an error when the element to be removed is not present, use the `set.discard(x)`

method.

>>> s = {'Alice', 'Bob', 'Carl'} >>> s.pop('Bob') Traceback (most recent call last): File "<pyshell#7>", line 1, in <module> s.pop('Bob') TypeError: pop() takes no arguments (1 given) >>> s.remove('Bob') >>> s {'Alice', 'Carl'}

## Return Value of Set pop()

The return value of `set.pop()`

is an `object`

that was stored as a set element. If the set is empty, it raises a `KeyError`

.

## How to Fix KeyError: ‘pop from an empty set’

? The `set.pop()`

method assumes that the set is non-empty. If you call the method on an empty set, it raises a `KeyError: 'pop from an empty set'`

. To fix this, make sure that the set is non-empty before calling the pop() method or use the `set.discard()`

method instead that doesn’t raise an error on a non-existent key.

>>> set().pop() Traceback (most recent call last): File "<pyshell#11>", line 1, in <module> set().pop() KeyError: 'pop from an empty set'

Here’s a code function `pop_no_error()`

that takes a set argument and pops an arbitrary element if one exists. Otherwise it returns `None`

:

>>> def pop_no_error(my_set): return my_set.pop() if my_set else None >>> pop_no_error({1, 2, 3}) 1 >>> pop_no_error(set()) >>>

## set.pop() vs set.remove()

The `set.pop()`

method is similar to the `set.remove()`

method. Both methods remove an element from the set. However, they’re quite different nonetheless.

? The difference between `set.pop()`

and `set.remove()`

is that the former** removes and returns an arbitrary element** whereas the latter

**.**

*removes a specific element and doesn’t return it*You can see this in the following example:

>>> s = {'Alice', 'Bob', 'Carl'} >>> s.remove('Carl') >>> s {'Alice', 'Bob'} >>> s.pop() 'Alice' >>> s {'Bob'}

Please note that the **remove **method has no return value but side effects—the element `'Carl'`

doesn’t appear in the set anymore. The **pop **method has a return value and has side effects as well—the element `'Alice'`

doesn’t appear in the set anymore.

## Python Set Pop Multiple Elements

**Problem Formulation**: Given a *set *and an integer *n*. You want to remove and return (“pop”) n random elements from the set. How do you accomplish this?

? To pop multiple elements from a set, use the list comprehension expression `[s.pop() for i in range(n)]`

. This assumes that the set has at least `n`

elements, otherwise a `KeyError`

will be raised.

Here’s the code:

s = {'Alice', 'Bob', 'Carl', 'Liz', 'Ann'} n = 3 def pop_n(s, n): '''Remove and return n elements from s''' if n>len(s): return list(s) return [s.pop() for i in range(n)] print(pop_n(s, n)) # ['Ann', 'Liz', 'Bob'] print(s) # {'Ann', 'Carl'}

## What is the Time Complexity of set.pop()?

The runtime complexity of the `set.pop()`

function on a set with *n* elements is * O(1)*. So, Python’s

`set.pop()`

method has *. It simply removes and returns the first element it encounters.*

**constant runtime complexity**You can see this in the following simple experiment where we run the set method multiple times for an increasing number of set elements.

I ran this experiment on my ** Acer Aspire 5 notebook** (I know) with

**(8th Gen) processor and 16GB of memory. Here’s the code of the experiment:**

*Intel Core i7*import matplotlib.pyplot as plt import time sizes = [i * 10**5 for i in range(1, 50)] runtimes = [] for size in sizes: s = set(range(size)) # Start track time ... t1 = time.time() s.pop() t2 = time.time() # ... end track time runtimes.append(t2-t1) plt.plot(sizes, runtimes) plt.ylabel('Runtime (s)') plt.xlabel('Set Size') plt.show()

## Other Python Set Methods

All set methods are called on a given set. For example, if you created a set `s = {1, 2, 3}`

, you’d call `s.clear()`

to remove all elements of the set. We use the term ** “this set”** to refer to the set on which the method is executed.

`add()` | Add an element to this set |

`clear()` | Remove all elements from this set |

`copy()` | Create and return a flat copy of this set |

`difference()` | Create and return a new set containing all elements of this set except the ones in the given set arguments. The resulting set has at most as many elements as any other. |

`difference_update()` | Remove all elements from this set that are members of any of the given set arguments. |

`discard()` | Remove an element from this set if it is a member, otherwise do nothing. |

`intersection()` | Create and return a new set that contains all elements that are members of all sets: this and the specified as well. . |

`intersection_update()` | Removes all elements from this set that are not members in all other specified sets. |

`isdisjoint()` | Return `True` if no element from this set is a member of any other specified set. Sets are disjoint if and only if their intersection is the empty set. |

`issubset()` | Return `True` if all elements of this set are members of the specified set argument. |

`issuperset()` | Return `True` if all elements of the specified set argument are members of this set. |

`pop()` | Remove and return a random element from this set. If the set is empty, it’ll raise a `KeyError` . |

`remove()` | Remove and return a specific element from this set as defined in the argument. If the set doesn’t contain the element, it’ll raise a `KeyError` . |

`symmetric_difference()` | Return a new set with elements in either this set or the specified set argument, but not elements that are members of both. |

`symmetric_difference_update()` | Replace this set with the symmetric difference, i.e., elements in either this set or the specified set argument, but not elements that are members of both. |

`union()` | Create and return a new set with all elements that are in this set, or in any of the specified set arguments. |

`update()` | Update this set with all elements that are in this set, or in any of the specified set arguments. The resulting set has at least as many elements as any other. |

While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.

To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.

His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.