If you read Python threads in dev communities, you often see snippets that use asterisks in surprising ways: passing lists into functions with a star, forwarding *args and **kwargs, or merging dictionaries on the fly. These patterns make your APIs flexible and your call sites clean, but they can be confusing until you see them side by side with examples.<\/p>\n\n\n\n
Hi everyone,<\/em> Python allows you to pass functions with a variable number of arguments instead of only what\u2019s hard-coded. This can be done with the unpacking operator After you understand the generic patterns, you can apply them to SDKs. With Cloudinary\u2019s Python SDK, it is common to compose upload options and transformation steps dynamically, then unpack them into a single call.<\/p>\n\n\n Packing lets you keep clean defaults while letting callers override exactly what they need. Unpacking ensures concise, readable calls. <\/p>\n\n\n\n
I keep seeing *args<\/code> and **kwargs<\/code> in function signatures and calls, along with sequence and dict unpacking in assignments and merges. Could someone explain clearly how to pack and unpack arguments in Python functions? I want to understand the syntax, common pitfalls, and best practices for writing flexible functions and forwarding parameters.<\/em><\/p>\n\n\n\nAnswer:<\/h2>\n\n\n\n
*<\/code>, letting you store arguments in a tuple, and looping over the arguments as an iterator. Let\u2019s break it down.<\/p>\n\n\n\n1) What packing and unpacking mean<\/h3>\n\n\n\n
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*args<\/code> and multiple keyword arguments into a dict via **kwargs<\/code>.<\/li>\n\n\n\n*<\/code> or a mapping into keyword arguments with **<\/code> at the call site or assignment.<\/li>\n<\/ul>\n\n\n\n2) Define flexible function signatures<\/h3>\n\n\n
def greet(greeting, *names, punctuation=\"!\"<\/span>):\n\u00a0 \u00a0 # names is a tuple of any extra positional args<\/span>\n\u00a0 \u00a0 joined = \", \"<\/span>.join(names) if<\/span> names else<\/span> \"there\"<\/span>\n\u00a0 \u00a0 return<\/span> f\"{greeting}, {joined}{punctuation}\"<\/span>\n\ndef configure(a, b=0<\/span>, *, verbose=False<\/span>, **options):\n\u00a0 \u00a0 # verbose is keyword-only, options captures extra keywords<\/span>\n\u00a0 \u00a0 return<\/span> a, b, verbose, options\n\ndef coords(x, y, \/, *, unit=\"px\"<\/span>):\n\u00a0 \u00a0 # x and y are positional-only, unit is keyword-only<\/span>\n\u00a0 \u00a0 return<\/span> f\"{x},{y} {unit}\"<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n3) Unpack in function calls<\/h3>\n\n\n
vals = [1<\/span>, 2<\/span>, 3<\/span>]\nprint<\/span>(sum(*[vals]))\u00a0 # same as sum(vals)<\/span>\n\npair = (5<\/span>, 7<\/span>)\nprint<\/span>(coords(*pair, unit=\"cm\"<\/span>))\u00a0 # expands to coords(5, 7, unit=\"cm\")<\/span>\n\npayload = {\"greeting\"<\/span>: \"Hello\"<\/span>, \"punctuation\"<\/span>: \"!!\"<\/span>}\nprint<\/span>(greet(**payload, *[\"Alice\"<\/span>, \"Bob\"<\/span>]))\u00a0 # mix unpacking in calls<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n4) Forwarding patterns<\/h3>\n\n\n
def logging_wrapper(func, *args, **kwargs):\n\u00a0 \u00a0 print<\/span>(\"Calling\"<\/span>, func.__name__, \"with\"<\/span>, args, kwargs)\n\u00a0 \u00a0 return<\/span> func(*args, **kwargs)\n\ndef base(*args, retries=3<\/span>, **kwargs):\n\u00a0 \u00a0 # Forward with defaults<\/span>\n\u00a0 \u00a0 return<\/span> do_work(*args, retries=retries, **kwargs)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n5) Unpacking in assignments and merges<\/h3>\n\n\n
# Starred assignment<\/span>\nfirst, *middle, last = [10<\/span>, 20<\/span>, 30<\/span>, 40<\/span>]\n# first=10, middle=[20, 30], last=40<\/span>\n\n# Dict merges and overrides<\/span>\ndefaults = {\"quality\"<\/span>: \"auto\"<\/span>, \"format\"<\/span>: \"jpg\"<\/span>}\noverrides = {\"format\"<\/span>: \"webp\"<\/span>, \"width\"<\/span>: 800<\/span>}\nmerged = {**defaults, **overrides}\n# merged == {\"quality\": \"auto\", \"format\": \"webp\", \"width\": 800}<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n6) Gotchas and best practices<\/h3>\n\n\n\n
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positional, *args, keyword-only, **kwargs<\/code>.<\/li>\n\n\n\n**kwargs<\/code> if a clear, explicit API is better. <\/strong>Use it when forwarding or supporting extensibility.<\/li>\n\n\n\n**kwargs<\/code> you honor and pass the rest through.<\/li>\n<\/ul>\n\n\n\nExample: Using packing and unpacking with the Cloudinary Python SDK<\/h3>\n\n\n\n
import cloudinary.uploader as<\/span> uploader\n\nDEFAULT_UPLOAD_OPTS = {\n\u00a0 \u00a0 \"folder\"<\/span>: \"samples\"<\/span>,\n\u00a0 \u00a0 \"overwrite\"<\/span>: True<\/span>,\n\u00a0 \u00a0 \"tags\"<\/span>: [\"demo\"<\/span>]\n}\n\nuser_overrides = {\n\u00a0 \u00a0 \"public_id\"<\/span>: \"avatar_123\"<\/span>,\n\u00a0 \u00a0 \"resource_type\"<\/span>: \"image\"<\/span>\n}\n\n# Merge defaults with user overrides<\/span>\nupload_opts = {**DEFAULT_UPLOAD_OPTS, **user_overrides}\n\n# Build a transformation pipeline dynamically<\/span>\nbase = {\"width\"<\/span>: 800<\/span>, \"height\"<\/span>: 800<\/span>, \"crop\"<\/span>: \"fill\"<\/span>}\nwatermark = {\"overlay\"<\/span>: \"logo\"<\/span>, \"gravity\"<\/span>: \"south_east\"<\/span>, \"x\"<\/span>: 15<\/span>, \"y\"<\/span>: 15<\/span>, \"opacity\"<\/span>: 70<\/span>}\nextra = {\"fetch_format\"<\/span>: \"auto\"<\/span>, \"quality\"<\/span>: \"auto\"<\/span>}\n\n# transformation can be a list of dicts<\/span>\ntransformations = [base, watermark, extra]\n\n# Unpack the dict as keyword args, pass the list directly<\/span>\nresult = uploader.upload(\n\u00a0 \u00a0 \"local\/path\/to\/image.jpg\"<\/span>,\n\u00a0 \u00a0 transformation=transformations,\n\u00a0 \u00a0 **upload_opts\n)\nprint<\/span>(result[\"secure_url\"<\/span>])<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\nTL;DR<\/h3>\n\n\n\n
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*args<\/code> packs extra positional args; **kwargs<\/code> packs extra keyword args.<\/li>\n\n\n\n*<\/code> and **<\/code> at call sites, in assignments, and for dict merges.<\/li>\n\n\n\n*args<\/code> and **kwargs<\/code> to keep wrappers flexible.<\/li>\n\n\n\nkwargs<\/code> you consume vs forward.<\/li>\n\n\n\nLearn More<\/h2>\n\n\n\n