Exercise 3 - LRU Cache


In exercise 2 we experimented with how and when Python automatically deallocates memory. This exercise will delve deeper into the automatic deallocation logic used in Python.

In Python, Garbage Collection is when the program identifies and releases blocks of memory that are no longer in use. The Python Garbage Collector (GC) operates while the program is running and is activated when the reference count of a particular object in memory reaches zero. The reference count increases when an object is given a new name or is placed in a container such as a tuple or a dictionary. Conversely, the reference count decreases when a name referring to the object is reassigned to a different object instead, or a variable referring to the object goes out of scope, or when a container referencing the object is destroyed.

The automatic garbage collection is really helpful, however, it may not clear all blocks of memory we would expect it to.

Symbolic information in native mode

When passing the --native flag to the run subcommand, Memray will collect information about the native call stack leading to each allocation, and it will dump it to the result file. This information is in a raw form where every call is identified by a number called the “instruction pointer” or “program counter”. When creating reports, Memray needs to convert these instruction pointers into human readable information, like the function name, the file name where that function is defined, and the line number within that file corresponding to the instruction.

This is particularly helpful when working with libraries written in a native language like C, C++, Fortran, or Rust (for instance functools, numpy, cryptography, etc).

Read more about how Memray resolves symbols in “native” mode here.

Working Through an Example

Let’s work through another example where data is held in memory longer than we may expect.


Let’s have a look at the example in lru_cache.py under exercise 3: can you spot any memory-related bugs in the code? Try running memray and generating a flamegraph — was the memory allocated the way you expected it to be?


Experiment with the code in lru_cache.py and try to get the peak memory usage down to 70MB. Test your solutions by running the unit test in tests/test_exercise_3.py and examine them with the help of memray reports.

Utilizing the Native mode

Let’s have a look at our flamegraph — we can see that the majority of the allocations come from the return statement in the factorial_plus() method. That’s quite odd, as the statement doesn’t look to be doing any memory heavy operations.


Let’s give the --native mode a go and see if we can uncover what might be causing the underlying memory-heavy operations. Can you spot anything new that might help us understand what’s causing such high memory usage?



Hint 1

The cache decorator works with methods that have hashable arguments - it caches the result of the decorated method per unique list of parameters. The results in the cache are kept alive until they age out (we have not set the size limit for our cache so this will never happen) of the cache or until the cache is cleared manually.

Let’s have another look at the method being cached:

def factorial_plus(self, n: int) -> int:
    return n * self.factorial_plus(n - 1) + self.inc if n else 1 + self.inc

How and which of the method calls be cached?

Hint 2

Remove the comment # pylint: disable=W1518 on line 17, and then run pylint to see another hint.


Toggle to see the sample solutions

There are many different approaches to fix this memory issue - here are a few of them:

  1. The cache decorator calls functools.lru_cache(maxsize=None). The lru_cache object itself stores (“memoizes”) the results, and retains references to all argument values passed to the decorated function in the cache. So, if we invoke such a decorated function with an object as a parameter, that object will persist in memory indefinitely, until the program terminates. If no other object ever compares equal to that object, we can never again get a cache hit for it, thereby squandering cache space. This scenario frequently arises when decorating a method, with the first parameter being self.

    One solution for this specific case involves utilizing a dedicated memoization method that stores the cache on the self object itself. This arrangement ensures that the cache is released alongside the object.

    class Algorithms:
        def __init__(self, inc: int):
            self.inc = inc
            self.factorial_plus = functools.cache(self._uncached_factorial_plus)
        def _uncached_factorial_plus(self, n: int) -> int:
            return n * self.factorial_plus(n - 1) + self.inc if n > 1 else 1 + self.inc
    def generate_factorial_plus_last_digit(plus_range: int, factorial_range: int):
        for i in range(plus_range):
            A = Algorithms(i)
            for j in range(factorial_range):
                yield A.factorial_plus(j) % 10

    Full code solution here.

  2. Another approach would be setting a maximum size for the cache. We can do that by passing an argument to @lru_cache decorator directly.


    @cache just wraps @lru_cache with some default arguments; we can only set the cache size ourselves if we use the @lru_cache decorator directly.

    def factorial_plus(self, n: int) -> int:
        return n * self.factorial_plus(n - 1) + self.inc if n else 1 + self.inc

    maxsize= here sets the maximum number of values stored in the cache.

  3. Finally, we can periodically manually invoke the cleanup of the cache. This can be done by calling Algorithms.factorial_plus.cache_clear()


The @functools.cache decorator is a powerful tool that can help make our programs much more efficient. It is crucial to fully understand how this decorator works before attempting to use it. By decorating an instance method, we have included the instance of this class (self) as part of the key to our cached data. This can easily lead to unexpected memory leaks when working with multiple instances of this class. That is because the LRU cache retains references to all of the arguments of the decorated function in its cache. Consequently, if we invoke such a decorated function with an object as an argument, that object will persist in memory indefinitely, or until the program terminates or the cache is cleared (reference counts in the GC for those cached objects are always > 0). If no other object instance ever compares equal to the one we’ve used as a cache key, we’ll never get a cache hit but are unnecessarily holding the object alive as a cache key, leading to unnecessary memory consumption.

In this tutorial we’ve learned to use Memray, either through manual inspection of Python scripts or via the pytest API. These methods are helpful tools for catching these, and other similar unexpected memory-related behaviors.

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