TopolearnEvery notebook surprise, from 'NameError' to stale results, comes from how cells and execution order work.
A notebook (.ipynb file) is a sequence of cells. A code cell holds code that you run with Shift+Enter; its output (printed text, the value of the last expression, a chart, a table) appears right below it and is saved in the file. A Markdown cell holds formatted text, headings, lists, links and LaTeX math, so a notebook can read like a report.
Code runs in a separate process called the kernel (for Python, an IPython kernel). The kernel keeps one shared namespace for the whole notebook: a variable defined in one cell is visible in every other cell run afterwards.
The number in brackets, [5], shows the order in which cells were executed. This order, not the position on the page, determines the kernel's state. You can run cell 7 and then cell 2, edit cell 2 and rerun it, or delete a cell whose variable still lives in memory. The result is hidden state: the notebook on screen no longer explains the results.
Habits that avoid trouble:
- Run cells top to bottom and keep the notebook runnable in that order.
- Before sharing, use Restart Kernel and Run All Cells; if anything fails, the notebook was relying on hidden state.
- Keep cells small and focused: load data, transform, plot.
Only the last expression of a cell is displayed automatically; use print() or display() for anything else.
Example
# Cell [1]
prices = [3.5, 4.0, 2.25]
# Cell [2]
total = sum(prices)
total # last expression is displayed: 9.75
# Cell [3] - later edited to: prices = [1, 2] and re-run
prices = [1, 2]
# Cell [2] is NOT re-run automatically, so `total` is stale:
print(total) # 9.75, although sum(prices) is now 3
print(sum(prices)) # 3
# Fix: Kernel > Restart Kernel and Run All CellsRemember
- Cells share one kernel namespace; execution order defines state
- Only the last expression of a cell is shown automatically
- Restart and Run All before trusting or sharing results
Watch out for
- Deleting a cell and assuming its variables are gone; they live in the kernel until restart.