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Explain Code Line by Line

Get a plain-English walkthrough of unfamiliar code, pitched at your experience level, with the key concepts called out.

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Fill in each variable and watch the prompt update. Anything still in {{braces}} needs your input before you use it.

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Programming language.

Your background, so the explanation fits.

The code snippet to explain.

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You are a patient programming tutor.

Explain the {{language}} code below to someone at this level: {{experience_level}}.

Format:
1. **Big picture** — one or two sentences on what the code does overall.
2. **Line by line** — walk through it in order. Group trivial lines together; spend more time on anything tricky.
3. **Key concepts** — list up to 3 concepts used here that are worth learning, with a one-sentence definition each.
4. **Watch out** — mention any bugs, edge cases, or risky patterns you notice. If there are none, say so.

Don't rewrite the code unless I ask. If something depends on context you can't see (like an imported function), say what you're assuming.

Code:
"""
{{code}}
"""
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Example

Example input

Filled prompt
You are a patient programming tutor.

Explain the Python code below to someone at this level: a beginner who knows variables and loops but not dictionaries.

Format:
1. **Big picture** — one or two sentences on what the code does overall.
2. **Line by line** — walk through it in order. Group trivial lines together; spend more time on anything tricky.
3. **Key concepts** — list up to 3 concepts used here that are worth learning, with a one-sentence definition each.
4. **Watch out** — mention any bugs, edge cases, or risky patterns you notice. If there are none, say so.

Don't rewrite the code unless I ask. If something depends on context you can't see (like an imported function), say what you're assuming.

Code:
"""
def word_counts(text):
    counts = {}
    for word in text.lower().split():
        counts[word] = counts.get(word, 0) + 1
    return sorted(counts.items(), key=lambda pair: pair[1], reverse=True)
"""

Example output

Illustrative example response — written by Promptly, not generated live

Big picture: This function counts how many times each word appears in a piece of text and returns the words sorted from most to least common.

Line by line:

  • def word_counts(text): — defines a function that takes one input, text.
  • counts = {} — creates an empty dictionary: a collection that maps keys (here, words) to values (here, counts).
  • for word in text.lower().split(): — lowercases the text so "The" and "the" match, then splits it on whitespace into a list of words, and loops over them.
  • counts[word] = counts.get(word, 0) + 1 — looks up the word's current count (or 0 if it's new) and adds 1.
  • return sorted(...) — turns the dictionary into a list of (word, count) pairs and sorts by the count (pair[1]), highest first.

Key concepts:

  • Dictionary — stores key/value pairs for fast lookup.
  • .get(key, default) — reads a value without crashing if the key is missing.
  • lambda — a tiny unnamed function, used here to tell sorted what to sort by.

Watch out: Punctuation stays attached to words, so "end." and "end" are counted separately.

Why it works

Stating the experience level is the most important part: the same code needs a very different explanation for a beginner than for a senior developer.

The fixed four-part format keeps answers consistent and skimmable, and the "Watch out" section turns a passive explanation into a mini code review. Asking the model to state its assumptions about code it can't see reduces confident-but-wrong guesses.

Techniques used