100 articles tagged #Python
Your Pandas pivot_table looks right but returns NaN in half the cells. The culprit is almost always an aggfunc receiving an empty group — here's how to diagnose and fix it cleanly.
Pandas groupby() is one of the most powerful tools for data aggregation, but subtle issues such as missing values, duplicate keys, incorrect aggregation functions, or dropped categories can silently produce incomplete results. Learn how to identify and prevent silent data loss in your groupby
If pandas.read_sql() is returning outdated, incomplete, or unexpected results, the problem often isn't Pandas itself. Database transactions, connection pooling, replicas, query caching, and isolation levels can all affect what your application reads. Learn how to identify and resolve the most common
Accidentally overwriting worksheets is one of the most common mistakes when using Python's openpyxl library. Whether you're appending reports, updating existing workbooks, or generating automated spreadsheets, understanding how openpyxl handles workbook saving can prevent costly data loss.
Many Python developers are surprised when simple calculations like 0.1 + 0.2 don't equal 0.3. The issue isn't a Python bug—it's how floating-point arithmetic works. Learn why floating-point precision errors occur, when to use Python's decimal module instead, and how to avoid costly numerical mistake
asyncio.gather() makes it easy to run multiple coroutines concurrently, but improperly handling its exceptions can leave failures unnoticed or cancel unrelated tasks. Learn how asyncio.gather() propagates exceptions, what return_exceptions=True actually does, and how to build reliable asynchronous
Custom validation in Django forms doesn't always execute the way developers expect. Misconfigured forms, incorrect validation flow, field-level errors, and improper form handling can cause clean() or clean_<field>() methods to appear as though they are being skipped. Learn how Django's validation
Python's csv.DictReader may appear to skip rows, produce malformed records, or fail unexpectedly when a CSV file is opened with the wrong text encoding. Learn how character encodings affect CSV parsing, how to diagnose encoding-related issues, and how to build reliable CSV import pipelines.
Running pd.merge() and getting way more rows than expected? When join keys aren't unique on both sides, Pandas does a cartesian product on matches — and the result can silently balloon your DataFrame. Here's how to diagnose and fix it.
Most data analysts use Jupyter Notebooks for one-off projects, but the same notebooks can become the foundation of a profitable recurring reporting business. Learn how to automate notebook execution, build client-ready reports, and transform your analytics expertise into a scalable subscription
Many data analysts build powerful Jupyter Notebook reports for internal projects, but few realize these notebooks can become profitable reporting services. Learn how to transform one-off analyses into scalable products, automate report generation, and build a recurring revenue business around data
Setting the values param in pd.crosstab() gives you aggregated cells, but the All margin row and column often show unexpected totals. Here's exactly why it happens and how to fix it for good.