Unit 5 · Moderate
Where Python stops being an exercise and starts touching real data. The unit adds a new category of failure: the input may be missing, malformed, or unavailable, and correct code has to survive that.
What a strong answer looks like
A strong Unit 5 answer says what happens when the data is absent or the wrong shape, not only what happens when everything works.
Know
A context manager opens a file and guarantees it is closed even if an error occurs partway through.
Apply
Always open files with a context manager, and read line by line when the file may be large.
Watch out
Reading an entire large file into memory when streaming would do.
Study move
Read a file line by line, count matching lines, and confirm the file closes on an error.
Know
CSV and JSON carry structure that plain text does not. Parsing with the right module avoids hand-splitting that breaks on quoted fields.
Apply
Use the CSV module rather than splitting on commas, because a quoted field may contain one.
Watch out
Splitting a CSV line on commas, which corrupts any row containing a quoted comma.
Study move
Parse a CSV row containing a quoted comma both ways and compare.
Know
An API returns structured data over the network, and both the request and the response can fail independently of your code being correct.
Apply
Check the response status before parsing, and handle the failure path explicitly.
Watch out
Assuming a successful response. Network calls fail routinely and silently corrupt downstream logic.
Study move
Describe every failure mode of one API call and what your code should do for each.
Know
Exception handling separates the expected path from the recovery path. Catching everything hides the bugs you needed to see.
Apply
Catch the specific exception you can actually recover from, and let the rest surface.
Watch out
A bare catch-all that swallows genuine errors and turns a crash into wrong output.
Study move
Write a read that handles a missing file specifically and lets other errors propagate.
Connections and techniques that receive extra attention in this unit.
Related topics some schools attach to this unit and others leave out. Covered on request rather than assumed.
Modeling drill
Write a script that reads a CSV of student scores and reports the average, handling a missing file and a malformed row differently.
