Perl Best Practices Reference Guide version 1.01.00
- 1. Code Layout 1. Brace and parenthesize in K&R style. 2. Separate your control keywords from the following opening bracket. 3. Don’t separate subroutine or variable names from the following opening bracket. 4. Don’t use unnecessary parentheses for builtins and ‘honorary’ builtins. 5. Separate complex keys or indices from their surrounding brackets. 6. Use whitespace to help binary operators stand out from their operands. 7. Place a semicolon after every statement. 8. Place a comma after every value in a multiline list. 9. Use 78-column lines. 10. Use four-column indentation levels. 11. Indent with spaces, not tabs. 12. Never place two statements on the same line. 13. Code in paragraphs. 14. Don’t cuddle an else. 15. Align corresponding items vertically. 16. Break long expressions before an operator. 17. Factor out long expressions in the middle of statements. 18. Always break a long expression at the operator of the lowest possi- ble precedence. 19. Break long assignments before the assignment operator. 20. Format cascaded ternary operators in columns. 21. Parenthesize long lists. 22. Enforce your chosen layout style mechanically. Naming Conventions 23. Use grammatical templates when forming identifiers. 24. Name booleans after their associated test. 25. Mark variables that store references with a _ref suffix. 26. Name arrays in the plural and hashes in the singular. 27. Use underscores to separate words in multiword identifiers. 28. Distinguish different program components by case. 29. Abbr idents by prefx. 30. Abbreviate only when the meaning remains unambiguous. 31. Avoid using inherently ambiguous words in names. 32. Prefix ‘for internal use only’ subroutines with an underscore. Values and Expressions 33. Use interpolating string delimiters only for strings that actually inter- polate. 34. Don’t use "" or '' for an empty string. 35. Don’t write one-character strings in visually ambiguous ways. 36. Use named character escapes instead of numeric escapes. 37. Use named constants, but don’t use constant. 38. Don’t pad decimal numbers with leading zeros. 39. Use underscores to improve the readability of long numbers. 40. Lay out multiline strings over multiple lines. 41. Use a heredoc when a multiline string exceeds two lines. 42. Use a ‘theredoc’ when a heredoc would compromise your indenta- tion. 43. Make every heredoc terminator a single uppercase identifier with a standard prefix. 44. When introducing a heredoc, quote the terminator. 45. Don’t use barewords. 46. Reserve => for pairs. 47. Don’t use commas to sequence statements. 48. Don’t mix high- and low-precedence booleans. 49. Parenthesize every raw list. 50. Use table-lookup to test for membership in lists of strings; use any() for membership of lists of anything else. Variables 51. Avoid using non-lexical variables. 52. Don’t use package variables in your own development. 53. If you’re forced to modify a package variable, localize it. 54. Initialize any variable you localize. 55. use English for the less familiar punctuation variables. 56. If you’re forced to modify a punctuation variable, localize it. 57. Don’t use the regex match variables. 58. Beware of any modification via $_. 59. Use negative indices when counting from the end of an array. 60. Take advantage of hash and array slicing. 61. Use a tabular layout for slices. 62. Factor large key or index lists out of their slices. Control Structures 63. Use block if, not postfix if. 64. Reserve postfix if for flow-of-control statements. 65. Don’t use postfix unless, for, while, or until. 66. Don’t use unless or until at all. 67. Avoid C-style for statements. 68. Avoid subscripting arrays or hashes within loops. 69. Never subscript more than once in a loop. 70. Use named lexicals as explicit for loop iterators. 71. Always declare a for loop iterator variable with my. 72. Use map instead of for when generating new lists from old. 73. Use grep and first instead of for when searching for values in a list. 74. Use for instead of map when transforming a list in place. 75. Use a subroutine call to factor out complex list transformations. 76. Never modify $_ in a list function. 77. Avoid cascading an if. 78. Use table look-up in preference to cascaded equality tests. 79. When producing a value, use tabular ternaries. 80. Don’t use do…while loops. 81. Reject as many iterations as possible, as early as possible. 82. Don’t contort loop structures just to consolidate control. 83. Use for and redo instead of an irregularly counted while. 84. Label every loop that is exited explicitly, and use the label with every next, last, or redo. Documentation 85. Distinguish user documentation from technical documentation. 86. Create standard POD templates for modules and applications. 87. Extend and customize your standard POD templates. 88. Put user documentation in source files. 89. Keep all user documentation in a single place within your source file. 90. Place POD as close as possible to the end of the file. 91. Subdivide your technical documentation appropriately. 92. Use block templates for major comments. 93. Use full-line comments to explain the algorithm. 94. Use end-of-line comments to point outsubtleties and oddities. 95. Comment anything that has puzzled or tricked you. 96. Consider whether it’s better to rewrite than to comment. 97. Use ‘invisible’ POD sections for longer technical discussions. 98. Check the spelling, syntax, and sanity of your documentation. Builtin Functions 99. Don’t recompute sort keys inside a sort. 100. Use reverse to reverse a list. 101. Use scalar reverse to reverse a scalar. 102. Use unpack to extract fixed-width fields. 103. Use split to extract simple variable-width fields. 104. Use Text::CSV_XS to extract complex variable-width fields. 105. Avoid string eval. 106. Consider building your sorting routines withSort::Maker. 107. Use 4-arg substr instead of lvalue substr. 108. Make appropriate use of lvalue values. 109. Use glob, not <…>. 110. Avoid a raw select for non-integer sleeps. 111. Always use a block with a map and grep. 112. Use the ‘non-builtin builtins’. Subroutines 113. Call subroutines with parentheses but without a leading &. 114. Don’t give subroutines the same names as built-in functions. 115. Always unpack @_ first. 116. Use a hash of named arguments for any subroutine that has more than three parameters. 117. Use definedness or existence to test for missing arguments. 118. Resolve any default argument values as soon as @_ is unpacked. 119. Always return scalar in scalar returns. 120. Make list-returning subroutines return the ‘obvious’ value in scalar context. 121. When there is no ‘obvious’ scalar context return value, consider Contextual::Return instead. 122. Don’t use subroutine prototypes. 123. Always return via an explicit return. 124. Use a bare return to return failure. I/O 125. Don’t use bareword filehandles. 126. Use indirect filehandles. 127. If you have to use a package filehandle, localize it first. Perl Best Practices Reference Guide
- 2. 128. Use either the IO::File module or the three-argument form of open. 129. Never open, close, or print to a file without checking the outcome. 130. Close filehandles explicitly, and as soon as possible. 131. Use while (<>), not for (<>). 132. Prefer line-based I/O to slurping. 133. Slurp a filehandle with a do block for purity. 134. Slurp a stream with Perl6::Slurp for power and simplicity. 135. Avoid using *STDIN, unless you really mean it. 136. Always put filehandles in braces within any print statement. 137. Always prompt for interactive input. 138. Don’t reinvent the standard test for interactivity. 139. Use the IO::Prompt module for prompting. 140. Always convey the progress of long non-interactive operations within interactive applications. 141. Consider using the Smart::Comments module to automate your progress indicators. 142. Avoid a raw select when setting autoflushes. References 143. Wherever possible, dereference with arrows. 144. Where prefix dereferencing is unavoidable, put braces around the reference. 145. Never use symbolic references. 146. Use weaken to prevent circular data structures from leaking memory. Regular Expressions 147. Always use the /x flag. 148. Always use the /m flag. 149. Use A and z as string boundary anchors. 150. Use z, not Z, to indicate ‘end of string’. 151. Always use the /s flag. 152. Consider mandating the Regexp::Autoflags module. 153. Use m{…} in preference to /…/ in multiline regexes. 154. Don’t use any delimiters other than /…/ or m{…}. 155. Prefer singular character classes to escaped metacharacters. 156. Prefer named characters to escaped metacharacters. 157. Prefer properties to enumerated character classes. 158. Consider matching arbitrary whitespace, rather than specific whites- pace characters. 159. Be specific when matching ‘as much as possible’. 160. Use capturing parentheses only when you intend to capture. 161. Use the numeric capture variables only when you’re sure that the preceding match succeeded. 162. Always give captured substrings proper names. 163. Tokenize input using the /gc flag. 164. Build regular expressions from tables. 165. Build complex regular expressions from simpler pieces. 166. Consider using Regexp::Common instead of writing your own regexes. 167. Always use character classes instead of single-character alternations. 168. Factor out common affixes from alternations. 169. Prevent useless backtracking. 170. Prefer fixed-string eq comparisons to fixed-pattern regex matches. Error Handling 171. Throw exceptions instead of returning special values or setting flags. 172. Make failed builtins throw exceptions too. 173. Make failures fatal in all contexts. 174. Be careful when testing for failure of the system builtin. 175. Throw exceptions on all failures, including recoverable ones. 176. Have exceptions report from the caller's location, not from the place where they were thrown. 177. Compose error messages in the recipient’s dialect. 178. Document every error message in the recipient’s dialect. 179. Use exception objects whenever failure data needs to be conveyed to a handler. 180. Use exception objects when error messages may change. 181. Use exception objects when two or more exceptions are related. 182. Catch exception objects in most-derived-first order. 183. Build exception classes automatically. 184. Unpack the exception variable in extended exception handlers. CommandLine Processing 185. Enforce a single consistent command-line structure. 186. Adhere to a standard set of conventions in your command-line syn- tax. 187. Standardize your meta-options. 188. Allow the same filename to be specified for both input and output. 189. Standardize on a single approach to command-line processing. 190. Ensure that your interface, run-time messages, and documentation remain consistent. 191. Factor out common command-line interface components into a shared module. Objects 192. Make object orientation a choice, not a default. 193. Choose object orientation using appropriate criteria. 194. Don’t use pseudohashes. 195. Don’t use restricted hashes. 196. Always use fully encapsulated objects. 197. Give every constructor the same standard name. 198. Don’t let a constructor clone objects. 199. Always provide a destructor for every inside-out class. 200. When creating methods, follow the general guidelines for subrou- tines. 201. Provide separate read and write accessors. 202. Don’t use lvalue accessors. 203. Don’t use the indirect object syntax. 204. Provide an optimal interface, rather than a minimal one. 205. Overload only the isomorphic operators of algebraic classes. 206. Always consider overloading the boolean, numeric, and string coer- cions of objects. Class Hierarchies 207. Don’t manipulate the list of base classes directly. 208. Use distributed encapsulated objects. 209. Never use the one-argument form of bless. 210. Pass constructor arguments as labeled values, using a hash refer- ence. 211. Distinguish arguments for base classes by class name as well. 212. Separate your construction, initialization, and destruction processes. 213. Build the standard class infrastructure automatically. 214. Use Class::Std to automate the deallocation of attribute data. 215. Have attributes initialized and verified automatically. 216. Specify coercions as :STRINGIFY, :NUMERIFY, and :BOOLIFY methods. 217. Use :CUMULATIVE methods instead of SUPER:: calls. 218. Don’t use AUTOLOAD(). Modules 219. Design the module’s interface first. 220. Place original code inline. Place duplicated code in a subroutine. Place duplicated subroutines in a module. 221. Use three-part version numbers. 222. Enforce your version requirements programmatically. 223. Export judiciously and, where possible, only by request. 224. Consider exporting declaratively. 225. Never make variables part of a module’s interface. 226. Build new module frameworks automatically. 227. Use core modules wherever possible. 228. Use CPAN modules where feasible. Testing and Debugging 229. Write the test cases first. 230. Standardize your tests with Test::Simple or Test::More. 231. Standardize your test suites with Test::Harness. 232. Write test cases that fail. 233. Test both the likely and the unlikely. 234. Add new test cases before you start debugging. 235. Always use strict. 236. Always turn on warnings explicitly. 237. Never assume that a warning-free compilation implies correctness. 238. Turn off strictures or warnings explicitly, selectively, and in the smallest possible scope. 239. Learn at least a subset of the perl debugger. 240. Use serialized warnings when debugging ‘manually’. 241. Consider using ‘smart comments’ when debugging, rather than warn statements. Miscellanea 242. Use a revision control system. 243. Integrate non-Perl code into your applications via the Inline:: modules. 244. Keep your configuration language uncomplicated. 245. Don’t use formats. 246. Don’t tie variables or filehandles. 247. Don’t be clever. 248. If you must rely on cleverness, encapsulate it. 249. Don’t optimize code — benchmark it. 250. Don’t optimize data structures — measure them. 251. Look for opportunities to use caches. 252. Automate your subroutine caching. 253. Benchmark any caching strategy you use. 254. Don’t optimize applications — profile them. 255. Be careful to preserve semantics when refactoring syntax. Perl Best Practices Reference Guide version 1.01.00. Perl Best Practices by Damian Conway © 2005 O’Reilly Media Inc., All rights reserved. Reference Guide by Johan Vromans © 2006 Squirrel Consultancy, All rights reserved.