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Diagrams: Reading Error Messages

Back to concept

Traceback Anatomy

Python error messages read from bottom to top. The last line tells you WHAT happened. The lines above tell you WHERE.

flowchart TD
    subgraph "Read the traceback bottom-to-top"
        LINE1["Traceback (most recent call last):"]
        LINE2["  File "main.py", line 10, in main"]
        LINE3["    result = calculate(x, y)"]
        LINE4["  File "math_utils.py", line 5, in calculate"]
        LINE5["    return x / y"]
        LINE6["ZeroDivisionError: division by zero"]
    end

    LINE6 --> WHAT["WHAT happened?<br/>ZeroDivisionError<br/>You divided by zero"]
    LINE5 --> WHERE_EXACT["WHERE exactly?<br/>Line: return x / y<br/>y must be 0"]
    LINE4 --> WHERE_FILE["WHICH file?<br/>math_utils.py, line 5<br/>in function calculate"]
    LINE2 --> WHO_CALLED["WHO called it?<br/>main.py, line 10<br/>in function main"]

    style LINE6 fill:#ff6b6b,stroke:#c0392b,color:#fff
    style LINE5 fill:#ffd43b,stroke:#f59f00,color:#000
    style LINE4 fill:#4a9eff,stroke:#2670c2,color:#fff
    style LINE2 fill:#51cf66,stroke:#27ae60,color:#fff
    style WHAT fill:#ff6b6b,stroke:#c0392b,color:#fff
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Common Error Types: Quick Diagnosis

When you see an error, match the type to its most likely cause.

flowchart TD
    ERR{"What error<br/>type did<br/>you get?"}

    ERR --> NAME["NameError"]
    NAME --> NAME_FIX["Did you spell the<br/>variable name right?<br/>Is it defined above<br/>this line?"]

    ERR --> TYPE["TypeError"]
    TYPE --> TYPE_FIX["Are you mixing types?<br/>&quot;5&quot; + 3 does not work.<br/>Did you call a<br/>non-function?"]

    ERR --> VALUE["ValueError"]
    VALUE --> VALUE_FIX["Right type but<br/>wrong content.<br/>int(&quot;hello&quot;) fails<br/>because &quot;hello&quot; is<br/>not a number."]

    ERR --> INDEX["IndexError"]
    INDEX --> INDEX_FIX["List too short.<br/>You asked for<br/>item 5 but only<br/>have 3 items."]

    ERR --> KEY["KeyError"]
    KEY --> KEY_FIX["Dict key missing.<br/>Check spelling.<br/>Use .get() to<br/>avoid crashes."]

    ERR --> SYNTAX["SyntaxError"]
    SYNTAX --> SYNTAX_FIX["Python cannot parse<br/>your code at all.<br/>Missing : or )<br/>or bad indentation."]

    ERR --> ATTR["AttributeError"]
    ATTR --> ATTR_FIX["That object does not<br/>have that method.<br/>Check the type with<br/>type(your_variable)."]

    style NAME fill:#4a9eff,stroke:#2670c2,color:#fff
    style TYPE fill:#ff922b,stroke:#e8590c,color:#fff
    style VALUE fill:#cc5de8,stroke:#9c36b5,color:#fff
    style INDEX fill:#51cf66,stroke:#27ae60,color:#fff
    style KEY fill:#ffd43b,stroke:#f59f00,color:#000
    style SYNTAX fill:#ff6b6b,stroke:#c0392b,color:#fff
    style ATTR fill:#20c997,stroke:#0ca678,color:#fff
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Error Message Reading Checklist

Follow these steps every time you get an error.

flowchart TD
    STEP1["Step 1:<br/>Look at the LAST LINE<br/>What is the error type<br/>and message?"]
    STEP1 --> STEP2["Step 2:<br/>Find the LINE NUMBER<br/>Which line of YOUR code<br/>caused it?"]
    STEP2 --> STEP3["Step 3:<br/>Look at THAT LINE<br/>What does it do?<br/>What values are involved?"]
    STEP3 --> STEP4{"Step 4:<br/>Can you fix it?"}
    STEP4 -->|Yes| FIX["Fix it and run again"]
    STEP4 -->|No| HELP["Add print() above<br/>that line to see<br/>what the values<br/>actually are"]
    HELP --> STEP3

    style STEP1 fill:#ff6b6b,stroke:#c0392b,color:#fff
    style STEP2 fill:#ffd43b,stroke:#f59f00,color:#000
    style STEP3 fill:#4a9eff,stroke:#2670c2,color:#fff
    style FIX fill:#51cf66,stroke:#27ae60,color:#fff
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