In Python threads and dev chats, one of the most common logic questions pops up when writing config handling or fallbacks: why does this expression return a string instead of a boolean, and why did my function never run? That often traces back to Python\u2019s or operator and how it evaluates expressions.<\/p>\n\n\n\n
I keep seeing code like Python\u2019s Truthy means any value that is not considered falsy. Falsy values include Once you understand or, you can use it to set safe defaults when preparing media for upload or delivery. For example, pick a default output format and quality from environment variables, then upload and build a delivery URL. <\/p>\n\n\nvalue = user_value or default_value<\/code>. I thought or returns a boolean, but in Python it seems to return one of the operands. Also, I hear it uses short-circuit evaluation and has tricky precedence with and and with things like NumPy arrays. How does the or operator work in Python? What are common patterns and pitfalls I should know, and how should I write safer code with it?<\/em><\/p>\n\n\n\nAnswer:<\/h2>\n\n\n\n
or<\/code> is a logical operator that returns one of its operands, not necessarily a boolean. It evaluates operands left to right and uses short-circuit evaluation.<\/p>\n\n\n\nWhat
or<\/code> returns<\/h3>\n\n\n\n\n
print<\/span>(0<\/span> or<\/span> \"fallback\"<\/span>)\u00a0 \u00a0 \u00a0 \u00a0 # \"fallback\"\u00a0 (0 is falsy)<\/span>\nprint<\/span>(\"hello\"<\/span> or<\/span> \"world\"<\/span>) \u00a0 \u00a0 # \"hello\" \u00a0 \u00a0 (first operand is truthy)<\/span>\nprint<\/span>([] or<\/span> {}) \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 # {}\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 (both falsy, returns last)<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\nFalse<\/code>, None<\/code>, 0<\/code>, empty sequences or collections like \"\"<\/code>, []<\/code>, ()<\/code>, {}<\/code>, and empty sets.<\/p>\n\n\n\nShort-circuit behavior<\/h3>\n\n\n\n
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def expensive():\n\u00a0 \u00a0 print<\/span>(\"expensive() called\"<\/span>)\n\u00a0 \u00a0 return<\/span> \"result\"<\/span>\n\nprint<\/span>(True<\/span> or<\/span> expensive()) \u00a0 # True, function never called<\/span>\nprint<\/span>(False<\/span> or<\/span> expensive())\u00a0 # \"result\", function is called<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\nOperator precedence<\/h3>\n\n\n\n
and<\/code> has higher precedence than or<\/code>. So a or b<\/code> and c<\/code> is parsed as a or (b and c)<\/code>. When in doubt, add parentheses.<\/p>\n\n\ndef expensive():\n\u00a0 \u00a0 print<\/span>(\"expensive() called\"<\/span>)\n\u00a0 \u00a0 return<\/span> \"result\"<\/span>\n\nprint<\/span>(True<\/span> or<\/span> expensive()) \u00a0 # True, function never called<\/span>\nprint<\/span>(False<\/span> or<\/span> expensive())\u00a0 # \"result\", function is called<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\nCommon patterns<\/h3>\n\n\n\n
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config_value = user_value<\/code> or default_value<\/code><\/li>\n\n\n\ndata = fetch() or compute_fallback(<\/code>) uses short-circuiting to avoid calling the fallback when not needed.<\/li>\n<\/ul>\n\n\n\nPitfalls and safer alternatives<\/h3>\n\n\n\n
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0<\/code> is a valid input, x = user_value<\/code> or 10<\/code> will replace 0<\/code>. Use explicit checks:x = 10 if user_value is None else user_value<\/code><\/li>\n<\/ul>\n\n\n\n\n
or<\/code> for defaults.<\/li>\n\n\n\nor<\/code> does not compare arrays and will raise errors about ambiguous truth values. Use bitwise | or any()\/all()<\/code>:<\/li>\n<\/ul>\n\n\n# Incorrect for arrays:<\/span>\n# if arr > 0 or arr < -1: ...<\/span>\n\n# Correct:<\/span>\nmask = (arr > 0<\/span>) | (arr < -1<\/span>)\nif<\/span> mask.any():\n\u00a0 \u00a0 ...\n\n# Or for pure booleans:<\/span>\nif<\/span> any(x > 0<\/span> for<\/span> x in arr):\n\u00a0 \u00a0 ...<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n\n
# Buggy intention: check if ext is \"jpg\" or \"jpeg\"<\/span>\nif<\/span> ext == \"jpg\"<\/span> or<\/span> \"jpeg\"<\/span>:\n\u00a0 \u00a0 pass\u00a0 # always truthy because \"jpeg\" is truthy<\/span>\n\n# Correct:<\/span>\nif<\/span> ext in {\"jpg\"<\/span>, \"jpeg\"<\/span>}:\n\u00a0 \u00a0 ...<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\nUsing
or<\/code> in a Python workflow with Cloudinary<\/h3>\n\n\n\nimport os\nfrom cloudinary import config, uploader, CloudinaryImage\n\n# Configure from env or use safe defaults with `or`<\/span>\ncloud_name = os.getenv(\"CLOUD_NAME\"<\/span>) or<\/span> \"demo\"<\/span>\napi_key = os.getenv(\"CLOUD_API_KEY\"<\/span>) or<\/span> \"123456\"<\/span>\napi_secret = os.getenv(\"CLOUD_API_SECRET\"<\/span>) or<\/span> \"abcxyz\"<\/span>\n\nconfig(cloud_name=cloud_name, api_key=api_key, api_secret=api_secret, secure=True<\/span>)\n\n# Choose format and quality with fallbacks<\/span>\nfmt = os.getenv(\"IMG_FORMAT\"<\/span>) or<\/span> \"webp\"<\/span>\nquality = int(os.getenv(\"IMG_QUALITY\"<\/span>) or<\/span> 80<\/span>)\n\n# Upload with options<\/span>\nres = uploader.upload(\n\u00a0 \u00a0 \"local-input.jpg\"<\/span>,\n\u00a0 \u00a0 folder=\"qa-or-operator\"<\/span>,\n\u00a0 \u00a0 format=fmt,\n\u00a0 \u00a0 quality=quality\n)\n\n# Build a delivery URL with transformations<\/span>\nurl = CloudinaryImage(res[\"public_id\"<\/span>]).build_url(\n\u00a0 \u00a0 format=fmt,\n\u00a0 \u00a0 transformation=[{\"width\"<\/span>: 800<\/span>, \"crop\"<\/span>: \"scale\"<\/span>}]\n)\nprint<\/span>(url)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n