For the most part, Python aims to be a clean and consistent language that avoids surprises. However, there are a few cases that can be confusing to newcomers.

Some of these cases are intentional but can be potentially surprising. Some could arguably be considered language warts. A new list is created once when the function is defined, and the same list is used in each successive call.

python - What is the pythonic way to avoid default parameters that are empty lists? - Stack Overflow

This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well. Create a new object each time the function is called, by using a default arg to signal that no argument was provided None is often a good choice. This is often done when writing a caching function. Another common source of confusion is the way Python binds its variables in closures or in the surrounding global scope.

A list containing five functions that each have their own closed-over i variable that multiplies their argument, producing:. Five functions are created; instead all of them just multiply x by 4. This means that the values of variables used in closures are looked up at the time the inner function is called.

Here, whenever any of the returned functions are called, the value of i is looked up in the surrounding scope at call time. By then, the loop has completed and i is left with its final value of 4.

Aoaal day trading stocks created with a lambda expression are in no way special, and in fact python default value none same exact behavior is exhibited by just using an ordinary def:.

The most general solution is arguably a bit of a hack. Sometimes you want your closures to behave this way.

Optional parameters in Python functions and their default values - Stack Overflow

Late binding is good in lots of situations. Looping to create unique functions is unfortunately a case where they can cause hiccups. By default, when executing Python code from files, the Python interpreter will automatically write a bytecode version of that file to disk, e.

Default Values for function parameters in Python - Stack Overflow

Theoretically, this behavior is on by default, for performance reasons. Without these bytecode files present, Python would re-generate the bytecode every time the file is loaded. Run that from the root directory of your project, and all. This opinionated guide exists to provide both novice and expert Python developers a best practice handbook to the installation, configuration, and usage of Python on a daily basis.

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