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Python Invalid Identifiers

Alstergymnasium Python
🧠 In Python, there’s a fixed list of keywords that cannot be used as identifiers (i.e., names for variables, functions, classes, etc.). They are reserved because they have a special meaning in the language core.

🛑 Invalid Identifiers: Python Keywords
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Here’s the complete list of reserved keywords in Python 3. You must not use these as names for variables, functions, or classes:

import keyword
print(keyword.kwlist)

📜 List of Keywords (Python 3.11+)
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Category Keywords
Control if, else, elif, while, for, break, continue, pass
Definitions def, class, lambda, return, yield
Logic & Comparison and, or, not, is, in
Error Handling try, except, finally, raise, assert
Import & Modules import, from, as, global, nonlocal
Data & Types True, False, None
Miscellaneous del, with, await, async, match, case

🚫 Examples of Invalid Identifiers
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def if():        # ❌ Invalid: "if" is a keyword
    pass

class None:      # ❌ Invalid: "None" is a special value

lambda = 5       # ❌ Invalid: "lambda" is a keyword

✅ Instead, use descriptive names like user_if, no_values, lambda_value.

✅ Valid Identifiers – Rules
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A valid identifier:

  • Starts with a letter or underscore (_)
  • Consists only of letters, digits, and underscores
  • Must not be a keyword

✔️ Examples:
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name = "Lena"
_count = 5
grade1 = 2

❌ Invalid Identifiers:
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1name = "Tom"     # ❌ starts with number
def = "Function"  # ❌ keyword

🧠 Tip for Students:
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If you’re unsure whether a word is allowed, you can check it:

import keyword
print(keyword.iskeyword("def"))  # Output: True