The uncomfortable truth
Ask any senior engineer how they spend their time and you'll hear the same thing: they read code much more than they write it. One estimate that circulates in the industry is 10:1 — ten lines read for every line written.
That sounds wasteful until you realize what reading buys you:
- You don't duplicate work that already exists.
- You don't violate invariants the codebase depends on.
- You can make a small change in a huge codebase without breaking everything.
- You can review PRs (like the one you reviewed last lesson) with real rigor.
Three modes of reading
Reading code isn't one thing. There are at least three different modes, and they call for different strategies:
1. Orientation read
"I've never seen this codebase. Where do things live?" Skim the README, the directory structure, the top-level config, the entrypoint file. Don't go deep — you're building a map, not a photograph.
2. Diagnostic read
"Something is broken. Where?" Start from the failure (a stack trace, a failing test, a user-visible bug) and work backwards through the code. Follow the data, not the control flow.
3. Impact read
"I want to change X. What breaks?" Start from what you want to change and work outwards — every caller, every test, every contract.
Most people try to do all three at once and get overwhelmed. Decide which mode you're in before you open a file.
Your pair-programmer cheat code
The AI tutor on the right of this page, and Devin in your capstone sessions, can both do one thing that's brutally useful: summarize code at the right level of abstraction. A good prompt:
"Give me a one-sentence summary of what this file does, then list the public functions it exports with one-line descriptions each."
Do that at the start of every orientation read and your pace will double.
Exercise
Pick any open-source repo you're curious about. Spend 10 minutes doing an orientation read. Then ask the tutor:
"Here's the README and folder structure of . What's the mental model a new contributor needs?"
Compare your mental model to the tutor's. Where did you disagree? Those are the gaps in your reading technique.