You say “import mymod”. Where does Python look? Check sys.path, a list of directory names (strings) where it searches. By default: – Current dir (empty string)– Standard library– site-packages The […] The post sys.path: How Python finds the modules you import appeared first on LernerPython.

In Python, import always defines a variable. But it only loads the module once. sys.modules, a dict, tracks already-loaded modules: – keys are strings, the module names– values are module […] The post sys.modules: How Python caches already-loaded modules appeared first on LernerPython.

“sys” is where Python keeps vital info about its runtime environment. So why import it? Not to load it; sys is loaded when Python starts. Rather, the import just defines […] The post Importing sys in Python: What the import statement really does appeared first on LernerPython.

Find the version of Python: To use it in a program, you want version_info: The post Python version at runtime: sys.version vs sys.version_info appeared first on LernerPython.

Want a Python Enum, but don’t care about the values? Use auto: You can now use Days.SUN and Days.MON. With auto, you care the Enum’s values are different, not what […] The post Python Enum auto(): Generating unique values automatically appeared first on LernerPython.

Want to iterate over a Python Enum? __members__ returns a dict-like object, and it supports items(): SUN: 1MON: 2TUE: 3[etc] The post Python Enum iteration: Looping over __members__.items() appeared first on LernerPython.

Normally, Python doesn’t enforce uniqueness on Enum values: The @unique decorator enforces: The post Python Enum duplicate values: Preventing aliases with @unique appeared first on LernerPython.

Want your Python Enum class to be sortable? The regular Enum doesn’t do it, but IntEnum does: Days.SUN < Days.THU # True!Days.MON > Days.WED # False! The post Python Enum sorting: Use IntEnum for comparable members appeared first on LernerPython.

Enums are a great addition to Python: They print nicer and are symbolic! But… not sortable (yet). The post Python enums: Symbolic constants that print nicely but don’t sort appeared first on LernerPython.

Can you get Python to enforce constants? Yes, with type checking: Ruff won’t complain. Instead, type it as Final and run mypy: mypy myprog.pymyprog.py:6: error: Cannot assign to final name […] The post Python constants: Blocking reassignment with Final and mypy appeared first on LernerPython.

About immutable vs. constant in Python: – Immutable: The value cannot change. If you own an immutable shirt, you can’t change its color, or add/remove a button.– Constant: Once assigned, […] The post Immutable vs. constant in Python: Why constants aren’t enforced appeared first on LernerPython.

Want the unique key-value pairs from two Python dicts? dict.items() returns a set-like object: This returns the unique key-value pairs: {(‘b’, 20), (‘a’, 10), (‘d’, 40), (‘c’, 30)} The post Python dict.items(): Set operations on key-value pairs appeared first on LernerPython.

Want the unique keys from two Python dicts? dict.keys() returns a set-like object, supporting set operators: The post Python dict.keys(): Using set operators on dictionary keys appeared first on LernerPython.

Differences between dict.pop and dict.popitem in Python: dict.pop: – removes a pair based on a key– returns removed value– optional 2nd arg is returned if key isn’t there dict.popitem: – […] The post dict.pop vs dict.popitem: Removing items from a Python dictionary appeared first on LernerPython.

To remove a key-value pair from a Python dict, use dict.pop: Or… pass a second argument to dict.pop. You’ll get it instead of an exception: The post Python dict.pop: Removing keys with a default fallback appeared first on LernerPython.

Merge two Python dicts with | which returns a new dict. Conflicts? The right-side dict gets priority: d1 | d2 # {‘a’: 10, ‘b’: 100, ‘c’: 200, ‘d’: 300}d2 | […] The post Merging Python dictionaries with |: Right side wins conflicts appeared first on LernerPython.

Iterating over a Python dict? Modifying existing values is fine, but adding or removing keys during iteration raises RuntimeError: The post Modifying a Python dict during iteration raises RuntimeError appeared first on LernerPython.

Want to iterate over a Python dict? The “items” method is your friend, returning a (key, value) tuple with each iteration: Remember: Insertion order determines iteration order. The post Python dict iteration: Using items to get keys and values appeared first on LernerPython.

Dictionaries in Python remember their insertion order, but equality ignores the ordering: The post Python dictionary equality: Insertion order doesn’t matter appeared first on LernerPython.

Want to create a dict of lists in Python? You can use defaultdict: The post Python dict of lists: Using defaultdict from collections appeared first on LernerPython.

Want to create a dict of lists in Python? Use setdefault: The post Python dicts of lists: Grouping values with setdefault appeared first on LernerPython.

Want to assign to a Python dict, but only if the key doesn’t yet exist? Use setdefault: The post Python dict.setdefault: Assign only when the key is missing appeared first on LernerPython.

Retrieve a value from a Python dict with the key: Request a key that doesn’t exist: Avoid that with dict.get, what I call “forgiving []” The post Python dictionaries: Avoiding KeyError with dict.get appeared first on LernerPython.

Want to check if a Python dict contains a key? Use “in” on the dict: ‘a’ in d # True‘x’ in d # False Searching in d.keys() is far slower: […] The post Python dict key lookup: Use in instead of d.keys() appeared first on LernerPython.

Want to create a Python dict? You have options; which is fastest? – Literal, with {‘a’:10, ‘b’:20}: 37.8 ns– Invoke dict(a=10, b=20): 58.5 ns– Invoke dict on a list of […] The post Python dict creation speed: Literal vs dict vs comprehension appeared first on LernerPython.

Trying to use .loc to retrieve a slice in Python Pandas? If the index repeats, you need to sort it: The post Pandas .loc slicing on a non-unique index: sort_index() first appeared first on LernerPython.

Retrieve from a Python Pandas data frame with .loc, which takes 2 arguments: 1. Row selector: index, list of indexes, or a mask index (i.e., booleans)2. Optional column selector: column […] The post Pandas .loc: Selecting rows, columns, and slices by label appeared first on LernerPython.

Filtering a Python Pandas data frame by the index? Pandas 3 adds pd.col, removing lambda from our .loc expression. Before: Now: Note: You cannot use pd.col with a series, only […] The post Pandas .loc filtering: Replacing lambda with pd.col appeared first on LernerPython.

To filter a Python Pandas data frame by rows, use the same syntax and rules as for series: The lambda still returns a boolean series. The expression can use any […] The post Pandas row filtering: Using df.loc with a lambda expression appeared first on LernerPython.

Why do I use loc+lambda to filter a Python Pandas series? 1. If earlier lines filter common values, later loc/lambda lines have less to filter — so queries run faster.2. […] The post Filtering a Pandas series with loc and lambda: Three benefits appeared first on LernerPython.