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Take for example a framework or a library you’re designing for other users, these users eventually will have a main function in their code in order to execute their app. If the user directly imports a sub-package of your library’s project then the init of that sub-package will be called(once) first of all. The same for the root package of the library, etc…
There are many times when you may want a code block to be executed without the existence of a main func, directly or not.
If you, as the developer of the imaginary library, import your library’s sub-package that has an init function, it will be called first and once, you don’t have a main func but you need to make sure that some variables, or a table, will be initialized before the calls of other functions.
A good thing to remember and not to worry about, is that:
the init always execute once per application.
init execution happens:
- right before the
initfunction of the “caller” package, - before the, optionally,
main func, - but after the package-level variables,
var = [...] or cost = [...],
When you import a package it will run all of its init functions, by order.
I’ll will give a very good example of an init function. It will add mime types to a standard go’s library named mime and a package-level function will use the mime standard package directly to get the custom mime types that are already be initialized at its init function:
package mime
import (
"mime"
"path/filepath"
)
var types = map[string]string{
".3dm": "x-world/x-3dmf",
".3dmf": "x-world/x-3dmf",
".7z": "application/x-7z-compressed",
".a": "application/octet-stream",
".aab": "application/x-authorware-bin",
".aam": "application/x-authorware-map",
".aas": "application/x-authorware-seg",
".abc": "text/vndabc",
".ace": "application/x-ace-compressed",
".acgi": "text/html",
".afl": "video/animaflex",
".ai": "application/postscript",
".aif": "audio/aiff",
".aifc": "audio/aiff",
".aiff": "audio/aiff",
".aim": "application/x-aim",
".aip": "text/x-audiosoft-intra",
".alz": "application/x-alz-compressed",
".ani": "application/x-navi-animation",
".aos": "application/x-nokia-9000-communicator-add-on-software",
".aps": "application/mime",
".apk": "application/vnd.android.package-archive",
".arc": "application/x-arc-compressed",
".arj": "application/arj",
".art": "image/x-jg",
".asf": "video/x-ms-asf",
".asm": "text/x-asm",
".asp": "text/asp",
".asx": "application/x-mplayer2",
".au": "audio/basic",
".avi": "video/x-msvideo",
".avs": "video/avs-video",
".bcpio": "application/x-bcpio",
".bin": "application/mac-binary",
".bmp": "image/bmp",
".boo": "application/book",
".book": "application/book",
".boz": "application/x-bzip2",
".bsh": "application/x-bsh",
".bz2": "application/x-bzip2",
".bz": "application/x-bzip",
".c++": "text/plain",
".c": "text/x-c",
".cab": "application/vnd.ms-cab-compressed",
".cat": "application/vndms-pkiseccat",
".cc": "text/x-c",
".ccad": "application/clariscad",
".cco": "application/x-cocoa",
".cdf": "application/cdf",
".cer": "application/pkix-cert",
".cha": "application/x-chat",
".chat": "application/x-chat",
".chrt": "application/vnd.kde.kchart",
".class": "application/java",
".com": "text/plain",
".conf": "text/plain",
".cpio": "application/x-cpio",
".cpp": "text/x-c",
".cpt": "application/mac-compactpro",
".crl": "application/pkcs-crl",
".crt": "application/pkix-cert",
".crx": "application/x-chrome-extension",
".csh": "text/x-scriptcsh",
".css": "text/css",
".csv": "text/csv",
".cxx": "text/plain",
".dar": "application/x-dar",
".dcr": "application/x-director",
".deb": "application/x-debian-package",
".deepv": "application/x-deepv",
".def": "text/plain",
".der": "application/x-x509-ca-cert",
".dif": "video/x-dv",
".dir": "application/x-director",
".divx": "video/divx",
".dl": "video/dl",
".dmg": "application/x-apple-diskimage",
".doc": "application/msword",
".dot": "application/msword",
".dp": "application/commonground",
".drw": "application/drafting",
".dump": "application/octet-stream",
".dv": "video/x-dv",
".dvi": "application/x-dvi",
".dwf": "drawing/x-dwf=(old)",
".dwg": "application/acad",
".dxf": "application/dxf",
".dxr": "application/x-director",
".el": "text/x-scriptelisp",
".elc": "application/x-bytecodeelisp=(compiled=elisp)",
".eml": "message/rfc822",
".env": "application/x-envoy",
".eps": "application/postscript",
".es": "application/x-esrehber",
".etx": "text/x-setext",
".evy": "application/envoy",
".exe": "application/octet-stream",
".f77": "text/x-fortran",
".f90": "text/x-fortran",
".f": "text/x-fortran",
".fdf": "application/vndfdf",
".fif": "application/fractals",
".fli": "video/fli",
".flo": "image/florian",
".flv": "video/x-flv",
".flx": "text/vndfmiflexstor",
".fmf": "video/x-atomic3d-feature",
".for": "text/x-fortran",
".fpx": "image/vndfpx",
".frl": "application/freeloader",
".funk": "audio/make",
".g3": "image/g3fax",
".g": "text/plain",
".gif": "image/gif",
".gl": "video/gl",
".gsd": "audio/x-gsm",
".gsm": "audio/x-gsm",
".gsp": "application/x-gsp",
".gss": "application/x-gss",
".gtar": "application/x-gtar",
".gz": "application/x-compressed",
".gzip": "application/x-gzip",
".h": "text/x-h",
".hdf": "application/x-hdf",
".help": "application/x-helpfile",
".hgl": "application/vndhp-hpgl",
".hh": "text/x-h",
".hlb": "text/x-script",
".hlp": "application/hlp",
".hpg": "application/vndhp-hpgl",
".hpgl": "application/vndhp-hpgl",
".hqx": "application/binhex",
".hta": "application/hta",
".htc": "text/x-component",
".htm": "text/html",
".html": "text/html",
".htmls": "text/html",
".htt": "text/webviewhtml",
".htx": "text/html",
".ice": "x-conference/x-cooltalk",
".ico": "image/x-icon",
".ics": "text/calendar",
".icz": "text/calendar",
".idc": "text/plain",
".ief": "image/ief",
".iefs": "image/ief",
".iges": "application/iges",
".igs": "application/iges",
".ima": "application/x-ima",
".imap": "application/x-httpd-imap",
".inf": "application/inf",
".ins": "application/x-internett-signup",
".ip": "application/x-ip2",
".isu": "video/x-isvideo",
".it": "audio/it",
".iv": "application/x-inventor",
".ivr": "i-world/i-vrml",
".ivy": "application/x-livescreen",
".jam": "audio/x-jam",
".jav": "text/x-java-source",
".java": "text/x-java-source",
".jcm": "application/x-java-commerce",
".jfif-tbnl": "image/jpeg",
".jfif": "image/jpeg",
".jnlp": "application/x-java-jnlp-file",
".jpe": "image/jpeg",
".jpeg": "image/jpeg",
".jpg": "image/jpeg",
".jps": "image/x-jps",
".js": "application/javascript",
".json": "application/json",
".jut": "image/jutvision",
".kar": "audio/midi",
".karbon": "application/vnd.kde.karbon",
".kfo": "application/vnd.kde.kformula",
".flw": "application/vnd.kde.kivio",
".kml": "application/vnd.google-earth.kml+xml",
".kmz": "application/vnd.google-earth.kmz",
".kon": "application/vnd.kde.kontour",
".kpr": "application/vnd.kde.kpresenter",
".kpt": "application/vnd.kde.kpresenter",
".ksp": "application/vnd.kde.kspread",
".kwd": "application/vnd.kde.kword",
".kwt": "application/vnd.kde.kword",
".ksh": "text/x-scriptksh",
".la": "audio/nspaudio",
".lam": "audio/x-liveaudio",
".latex": "application/x-latex",
".lha": "application/lha",
".lhx": "application/octet-stream",
".list": "text/plain",
".lma": "audio/nspaudio",
".log": "text/plain",
".lsp": "text/x-scriptlisp",
".lst": "text/plain",
".lsx": "text/x-la-asf",
".ltx": "application/x-latex",
".lzh": "application/octet-stream",
".lzx": "application/lzx",
".m1v": "video/mpeg",
".m2a": "audio/mpeg",
".m2v": "video/mpeg",
".m3u": "audio/x-mpegurl",
".m": "text/x-m",
".man": "application/x-troff-man",
".manifest": "text/cache-manifest",
".map": "application/x-navimap",
".mar": "text/plain",
".mbd": "application/mbedlet",
".mc$": "application/x-magic-cap-package-10",
".mcd": "application/mcad",
".mcf": "text/mcf",
".mcp": "application/netmc",
".me": "application/x-troff-me",
".mht": "message/rfc822",
".mhtml": "message/rfc822",
".mid": "application/x-midi",
".midi": "application/x-midi",
".mif": "application/x-frame",
".mime": "message/rfc822",
".mjf": "audio/x-vndaudioexplosionmjuicemediafile",
".mjpg": "video/x-motion-jpeg",
".mm": "application/base64",
".mme": "application/base64",
".mod": "audio/mod",
".moov": "video/quicktime",
".mov": "video/quicktime",
".movie": "video/x-sgi-movie",
".mp2": "audio/mpeg",
".mp3": "audio/mpeg3",
".mp4": "video/mp4",
".mpa": "audio/mpeg",
".mpc": "application/x-project",
".mpe": "video/mpeg",
".mpeg": "video/mpeg",
".mpg": "video/mpeg",
".mpga": "audio/mpeg",
".mpp": "application/vndms-project",
".mpt": "application/x-project",
".mpv": "application/x-project",
".mpx": "application/x-project",
".mrc": "application/marc",
".ms": "application/x-troff-ms",
".mv": "video/x-sgi-movie",
".my": "audio/make",
".mzz": "application/x-vndaudioexplosionmzz",
".nap": "image/naplps",
".naplps": "image/naplps",
".nc": "application/x-netcdf",
".ncm": "application/vndnokiaconfiguration-message",
".nif": "image/x-niff",
".niff": "image/x-niff",
".nix": "application/x-mix-transfer",
".nsc": "application/x-conference",
".nvd": "application/x-navidoc",
".o": "application/octet-stream",
".oda": "application/oda",
".odb": "application/vnd.oasis.opendocument.database",
".odc": "application/vnd.oasis.opendocument.chart",
".odf": "application/vnd.oasis.opendocument.formula",
".odg": "application/vnd.oasis.opendocument.graphics",
".odi": "application/vnd.oasis.opendocument.image",
".odm": "application/vnd.oasis.opendocument.text-master",
".odp": "application/vnd.oasis.opendocument.presentation",
".ods": "application/vnd.oasis.opendocument.spreadsheet",
".odt": "application/vnd.oasis.opendocument.text",
".oga": "audio/ogg",
".ogg": "audio/ogg",
".ogv": "video/ogg",
".omc": "application/x-omc",
".omcd": "application/x-omcdatamaker",
".omcr": "application/x-omcregerator",
".otc": "application/vnd.oasis.opendocument.chart-template",
".otf": "application/vnd.oasis.opendocument.formula-template",
".otg": "application/vnd.oasis.opendocument.graphics-template",
".oth": "application/vnd.oasis.opendocument.text-web",
".oti": "application/vnd.oasis.opendocument.image-template",
".otm": "application/vnd.oasis.opendocument.text-master",
".otp": "application/vnd.oasis.opendocument.presentation-template",
".ots": "application/vnd.oasis.opendocument.spreadsheet-template",
".ott": "application/vnd.oasis.opendocument.text-template",
".p10": "application/pkcs10",
".p12": "application/pkcs-12",
".p7a": "application/x-pkcs7-signature",
".p7c": "application/pkcs7-mime",
".p7m": "application/pkcs7-mime",
".p7r": "application/x-pkcs7-certreqresp",
".p7s": "application/pkcs7-signature",
".p": "text/x-pascal",
".part": "application/pro_eng",
".pas": "text/pascal",
".pbm": "image/x-portable-bitmap",
".pcl": "application/vndhp-pcl",
".pct": "image/x-pict",
".pcx": "image/x-pcx",
".pdb": "chemical/x-pdb",
".pdf": "application/pdf",
".pfunk": "audio/make",
".pgm": "image/x-portable-graymap",
".pic": "image/pict",
".pict": "image/pict",
".pkg": "application/x-newton-compatible-pkg",
".pko": "application/vndms-pkipko",
".pl": "text/x-scriptperl",
".plx": "application/x-pixclscript",
".pm4": "application/x-pagemaker",
".pm5": "application/x-pagemaker",
".pm": "text/x-scriptperl-module",
".png": "image/png",
".pnm": "application/x-portable-anymap",
".pot": "application/mspowerpoint",
".pov": "model/x-pov",
".ppa": "application/vndms-powerpoint",
".ppm": "image/x-portable-pixmap",
".pps": "application/mspowerpoint",
".ppt": "application/mspowerpoint",
".ppz": "application/mspowerpoint",
".pre": "application/x-freelance",
".prt": "application/pro_eng",
".ps": "application/postscript",
".psd": "application/octet-stream",
".pvu": "paleovu/x-pv",
".pwz": "application/vndms-powerpoint",
".py": "text/x-scriptphyton",
".pyc": "application/x-bytecodepython",
".qcp": "audio/vndqcelp",
".qd3": "x-world/x-3dmf",
".qd3d": "x-world/x-3dmf",
".qif": "image/x-quicktime",
".qt": "video/quicktime",
".qtc": "video/x-qtc",
".qti": "image/x-quicktime",
".qtif": "image/x-quicktime",
".ra": "audio/x-pn-realaudio",
".ram": "audio/x-pn-realaudio",
".rar": "application/x-rar-compressed",
".ras": "application/x-cmu-raster",
".rast": "image/cmu-raster",
".rexx": "text/x-scriptrexx",
".rf": "image/vndrn-realflash",
".rgb": "image/x-rgb",
".rm": "application/vndrn-realmedia",
".rmi": "audio/mid",
".rmm": "audio/x-pn-realaudio",
".rmp": "audio/x-pn-realaudio",
".rng": "application/ringing-tones",
".rnx": "application/vndrn-realplayer",
".roff": "application/x-troff",
".rp": "image/vndrn-realpix",
".rpm": "audio/x-pn-realaudio-plugin",
".rt": "text/vndrn-realtext",
".rtf": "text/richtext",
".rtx": "text/richtext",
".rv": "video/vndrn-realvideo",
".s": "text/x-asm",
".s3m": "audio/s3m",
".s7z": "application/x-7z-compressed",
".saveme": "application/octet-stream",
".sbk": "application/x-tbook",
".scm": "text/x-scriptscheme",
".sdml": "text/plain",
".sdp": "application/sdp",
".sdr": "application/sounder",
".sea": "application/sea",
".set": "application/set",
".sgm": "text/x-sgml",
".sgml": "text/x-sgml",
".sh": "text/x-scriptsh",
".shar": "application/x-bsh",
".shtml": "text/x-server-parsed-html",
".sid": "audio/x-psid",
".skd": "application/x-koan",
".skm": "application/x-koan",
".skp": "application/x-koan",
".skt": "application/x-koan",
".sit": "application/x-stuffit",
".sitx": "application/x-stuffitx",
".sl": "application/x-seelogo",
".smi": "application/smil",
".smil": "application/smil",
".snd": "audio/basic",
".sol": "application/solids",
".spc": "text/x-speech",
".spl": "application/futuresplash",
".spr": "application/x-sprite",
".sprite": "application/x-sprite",
".spx": "audio/ogg",
".src": "application/x-wais-source",
".ssi": "text/x-server-parsed-html",
".ssm": "application/streamingmedia",
".sst": "application/vndms-pkicertstore",
".step": "application/step",
".stl": "application/sla",
".stp": "application/step",
".sv4cpio": "application/x-sv4cpio",
".sv4crc": "application/x-sv4crc",
".svf": "image/vnddwg",
".svg": "image/svg+xml",
".svr": "application/x-world",
".swf": "application/x-shockwave-flash",
".t": "application/x-troff",
".talk": "text/x-speech",
".tar": "application/x-tar",
".tbk": "application/toolbook",
".tcl": "text/x-scripttcl",
".tcsh": "text/x-scripttcsh",
".tex": "application/x-tex",
".texi": "application/x-texinfo",
".texinfo": "application/x-texinfo",
".text": "text/plain",
".tgz": "application/gnutar",
".tif": "image/tiff",
".tiff": "image/tiff",
".tr": "application/x-troff",
".tsi": "audio/tsp-audio",
".tsp": "application/dsptype",
".tsv": "text/tab-separated-values",
".turbot": "image/florian",
".txt": "text/plain",
".uil": "text/x-uil",
".uni": "text/uri-list",
".unis": "text/uri-list",
".unv": "application/i-deas",
".uri": "text/uri-list",
".uris": "text/uri-list",
".ustar": "application/x-ustar",
".uu": "text/x-uuencode",
".uue": "text/x-uuencode",
".vcd": "application/x-cdlink",
".vcf": "text/x-vcard",
".vcard": "text/x-vcard",
".vcs": "text/x-vcalendar",
".vda": "application/vda",
".vdo": "video/vdo",
".vew": "application/groupwise",
".viv": "video/vivo",
".vivo": "video/vivo",
".vmd": "application/vocaltec-media-desc",
".vmf": "application/vocaltec-media-file",
".voc": "audio/voc",
".vos": "video/vosaic",
".vox": "audio/voxware",
".vqe": "audio/x-twinvq-plugin",
".vqf": "audio/x-twinvq",
".vql": "audio/x-twinvq-plugin",
".vrml": "application/x-vrml",
".vrt": "x-world/x-vrt",
".vsd": "application/x-visio",
".vst": "application/x-visio",
".vsw": "application/x-visio",
".w60": "application/wordperfect60",
".w61": "application/wordperfect61",
".w6w": "application/msword",
".wav": "audio/wav",
".wb1": "application/x-qpro",
".wbmp": "image/vnd.wap.wbmp",
".web": "application/vndxara",
".wiz": "application/msword",
".wk1": "application/x-123",
".wmf": "windows/metafile",
".wml": "text/vnd.wap.wml",
".wmlc": "application/vnd.wap.wmlc",
".wmls": "text/vnd.wap.wmlscript",
".wmlsc": "application/vnd.wap.wmlscriptc",
".word": "application/msword",
".wp5": "application/wordperfect",
".wp6": "application/wordperfect",
".wp": "application/wordperfect",
".wpd": "application/wordperfect",
".wq1": "application/x-lotus",
".wri": "application/mswrite",
".wrl": "application/x-world",
".wrz": "model/vrml",
".wsc": "text/scriplet",
".wsrc": "application/x-wais-source",
".wtk": "application/x-wintalk",
".x-png": "image/png",
".xbm": "image/x-xbitmap",
".xdr": "video/x-amt-demorun",
".xgz": "xgl/drawing",
".xif": "image/vndxiff",
".xl": "application/excel",
".xla": "application/excel",
".xlb": "application/excel",
".xlc": "application/excel",
".xld": "application/excel",
".xlk": "application/excel",
".xll": "application/excel",
".xlm": "application/excel",
".xls": "application/excel",
".xlt": "application/excel",
".xlv": "application/excel",
".xlw": "application/excel",
".xm": "audio/xm",
".xml": "text/xml",
".xmz": "xgl/movie",
".xpix": "application/x-vndls-xpix",
".xpm": "image/x-xpixmap",
".xsr": "video/x-amt-showrun",
".xwd": "image/x-xwd",
".xyz": "chemical/x-pdb",
".z": "application/x-compress",
".zip": "application/zip",
".zoo": "application/octet-stream",
".zsh": "text/x-scriptzsh",
".docx": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
".docm": "application/vnd.ms-word.document.macroEnabled.12",
".dotx": "application/vnd.openxmlformats-officedocument.wordprocessingml.template",
".dotm": "application/vnd.ms-word.template.macroEnabled.12",
".xlsx": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
".xlsm": "application/vnd.ms-excel.sheet.macroEnabled.12",
".xltx": "application/vnd.openxmlformats-officedocument.spreadsheetml.template",
".xltm": "application/vnd.ms-excel.template.macroEnabled.12",
".xlsb": "application/vnd.ms-excel.sheet.binary.macroEnabled.12",
".xlam": "application/vnd.ms-excel.addin.macroEnabled.12",
".pptx": "application/vnd.openxmlformats-officedocument.presentationml.presentation",
".pptm": "application/vnd.ms-powerpoint.presentation.macroEnabled.12",
".ppsx": "application/vnd.openxmlformats-officedocument.presentationml.slideshow",
".ppsm": "application/vnd.ms-powerpoint.slideshow.macroEnabled.12",
".potx": "application/vnd.openxmlformats-officedocument.presentationml.template",
".potm": "application/vnd.ms-powerpoint.template.macroEnabled.12",
".ppam": "application/vnd.ms-powerpoint.addin.macroEnabled.12",
".sldx": "application/vnd.openxmlformats-officedocument.presentationml.slide",
".sldm": "application/vnd.ms-powerpoint.slide.macroEnabled.12",
".thmx": "application/vnd.ms-officetheme",
".onetoc": "application/onenote",
".onetoc2": "application/onenote",
".onetmp": "application/onenote",
".onepkg": "application/onenote",
".xpi": "application/x-xpinstall",
}
func init() {
for ext, typ := range types {
// skip errors
mime.AddExtensionType(ext, typ)
}
}
// typeByExtension returns the MIME type associated with the file extension ext.
// The extension ext should begin with a leading dot, as in ".html".
// When ext has no associated type, typeByExtension returns "".
//
// Extensions are looked up first case-sensitively, then case-insensitively.
//
// The built-in table is small but on unix it is augmented by the local
// system's mime.types file(s) if available under one or more of these
// names:
//
// /etc/mime.types
// /etc/apache2/mime.types
// /etc/apache/mime.types
//
// On Windows, MIME types are extracted from the registry.
//
// Text types have the charset parameter set to "utf-8" by default.
func TypeByExtension(fullfilename string) string {
ext := filepath.Ext(fullfilename)
typ := mime.TypeByExtension(ext)
// mime.TypeByExtension returns as text/plain; | charset=utf-8 the static .js (not always)
if ext == ".js" && (typ == "text/plain" || typ == "text/plain; charset=utf-8") {
if ext == ".js" {
typ = "application/javascript"
}
}
return typ
}
Hope that helped you and other users, don’t hesitate to post again if you have more questions!
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