Don’t believe everything you learn - PHPSouthCoast

A minute always has 60 seconds, right? Wrong! Ok, but a country always has a capital city, right? Wrong again!

Come and join us for a lighthearted talk about some of the common falsehoods that developers believe which cause issues in real, production systems. We’ll cover the classics like timezones and names, as well as some new contenders such as geography and software versions.

Transcript

1. Don’t believe everything you learn @mheap at #phpsc17
2. Things we believe are true
3. Things we believe are true But they’re not
4. Falsehoods programmers believe about X
5. Michael
6. Time
7. @mheap The number of days in every month starts with a 3, so either 30 or 31?
8. @mheap The number of days in every month starts with a 3, so either 30 or 31? February has 28
9. #phpsc17 Each month always has the same number of days every year though?
10. #phpsc17 Each month always has the same number of days every year though? February has 29 in leap years
11. @mheap But we can calculate leap years. They’re every four years
12. @mheap But we can calculate leap years. They’re every four years Unless it’s divisible by 100
13. #phpsc17 But we can calculate leap years. They’re every four years (Unless it’s divisible by 100)
14. #phpsc17 But we can calculate leap years. They’re every four years (Unless it’s divisible by 100) If it divides by 100 and 400, it is a leap year
15. @mheap But that rule holds all the time
16. @mheap But that rule holds all the time Unless you’re Sweden
17. #phpsc17 February is 28 or 29 days February the 30th doesn’t exist
18. #phpsc17 February is 28 or 29 days February the 30th doesn’t exist Unless you’re Sweden
19. #phpsc17 February is 28 or 29 days February the 30th doesn’t exist
20. #phpsc17 February is 28 or 29 days February the 30th doesn’t exist Or the Soviet Union
21. @mheap A month starts and ends in the same year
22. @mheap A month starts and ends in the same year Not in ancient Rome
23. @mheap A month starts and ends in the same year
24. @mheap A month starts and ends in the same year Or a modern Ethiopia
25. #phpsc17 Excluding DST, time always goes forwards
26. #phpsc17 Excluding DST, time always goes forwards Shanghai, 1927 1927-12-31 23:54:08 happened twice
27. Modern Time
28. @mheap The day before Saturday is always Friday
29. @mheap The day before Saturday is always Friday Samoa, 2011
30. #phpsc17 The offsets between two timezones will always be the same
31. #phpsc17 The offsets between two timezones will always be the same 10 changes in 2016 2 changes so far in 2017
32. @mheap Timezones are always +/- an integer number of hours
33. @mheap Timezones are always +/- an integer number of hours Afghanistan, +04:30 New Zealand (Chatham Islands), +12:45
34. #phpsc17 15 minutes is safe though?
35. #phpsc17 15 minutes is safe though? UTC-00:25:21 - Dublin Mean Time
36. @mheap If they do change, there’ll be plenty of notice
37. @mheap If they do change, there’ll be plenty of notice North Korea, 2015 8 days notice of a 30 minute change
38. Audience Participation
39. @mheap There are always 24 hours in a day
40. @mheap There are always 24 hours in a day Daylight Savings Time Means 23 or 25 hours in a day
41. #phpsc17 Is the time 23:59:60 invalid?
42. #phpsc17 Is the time 23:59:60 invalid? Leap seconds A minute can have 61 seconds
43. Names HELLO , M Y N AM E IS
44. @mheap A child will have the same surname as either their mother or father (or both)
45. @mheap A child will have the same surname as either their mother or father (or both) Not in Iceland
46. #phpsc17 People have at least 2 names
47. #phpsc17 People have at least 2 names Teller (of Penn & Teller fame) Akihito (Emperor of Japan)
48. @mheap People’s names do not contain numbers
49. @mheap People’s names do not contain numbers John 5
50. #phpsc17 People’s names do not contain numbers
51. #phpsc17 People’s names do not contain numbers K8lyn? (Kaitlyn)
52. #phpsc17 People’s names do not contain numbers
53. #phpsc17 People’s names do not contain numbers KVIIIlyn
54. @mheap For every rule, there are guaranteed to be exceptions
55. @mheap For every rule, there are guaranteed to be exceptions No spaces Van Buren
56. @mheap For every rule, there are guaranteed to be exceptions
57. @mheap For every rule, there are guaranteed to be exceptions Only letters O’Reilly
58. @mheap For every rule, there are guaranteed to be exceptions
59. @mheap For every rule, there are guaranteed to be exceptions One capital letter McDonald
60. Audience Participation HELLO , M Y N AM E IS
61. @mheap A name must contain one or more alphanumeric characters
62. @mheap A name must contain one or more alphanumeric characters ⽑毛泽东
63. #phpsc17 A name must contain one or more unicode characters
64. #phpsc17 A name must contain one or more unicode characters
65. @mheap I can safely assume that this list of bad words has no names in it
66. @mheap I can safely assume that this list of bad words has no names in it Ever been to Middlesex?
67. @mheap I can safely assume that this list of bad words has no names in it
68. @mheap I can safely assume that this list of bad words has no names in it Eaten shittake mushrooms?
69. @mheap I can safely assume that this list of bad words has no names in it
70. @mheap I can safely assume that this list of bad words has no names in it Been named Craig Cockburn?
71. @mheap I can safely assume that this list of bad words has no names in it
72. @mheap I can safely assume that this list of bad words has no names in it Linda Callahan?
73. @mheap I can safely assume that this list of bad words has no names in it
74. #phpsc17 What about less obvious ones?
75. #phpsc17 What about less obvious ones? Super Bowl XXX
76. @mheap Did you hear about planning permission emails bouncing?
77. @mheap Did you hear about planning permission emails bouncing? Erection of a structure
78. #phpsc17 The sad story of
79. #phpsc17 The sad story of Christopher Null
80. Addresses
81. #phpsc17 A building number is numeric
82. #phpsc17 A building number is numeric 1A Egmont Road, Middlesbrough, TS4 2HT
83. @mheap No buildings are numbered zero
84. @mheap No buildings are numbered zero 0 Egmont Road, Middlesbrough, TS4 2HT
85. #phpsc17 If the addresses on the left are even, addresses on the right must be odd
86. #phpsc17 If the addresses on the left are even, addresses on the right must be odd Seine-Saint-Denis, France Even numbers on both sides, no odds
87. @mheap You can omit leading zeros
88. @mheap You can omit leading zeros 101 Alma St, Palo Alto Apartments 1 and 001 on different floors.
89. #phpsc17 A street name won't include a number
90. #phpsc17 A street name won't include a number 8 Seven Gardens Burgh, Woodbridge, IP13 6SU
91. @mheap You wouldn’t name a town Street
92. @mheap You wouldn’t name a town Street Street, Somerset, UK
93. #phpsc17 You can’t have two towns/cities with the same name in a country
94. #phpsc17 You can’t have two towns/cities with the same name in a country The UK has three places named Newport
95. @mheap Those cities don’t have duplicate street names
96. @mheap Those cities don’t have duplicate street names 10 High Street, Newport, PO30 1SS 10 High Street, Newport, NP20 1FQ 10 High Street, Newport, TF10 7AN
97. #phpsc17 Street names will be reasonably short
98. #phpsc17 Street names will be reasonably short Aleja Alije Izetbegovića Prvig Predsjednika Predsjedništva Republika Bosna i Hercegovina
99. @mheap A building has a single postcode
100. @mheap A building has a single postcode DVLA Swansea: V5Cs are processed at SA99 1BA, Driving licences at SA99 1AB
101. #phpsc17 Addresses are a fixed point in space
102. #phpsc17 Addresses are a fixed point in space BF1 4FB
103. @mheap Concrete buildings are a fixed point in space
104. @mheap Concrete buildings are a fixed point in space In Zürich, a 6200 ton building was moved by 60 meters to make way for railway tracks
105. Audience Participation
106. #phpsc17 Do all countries have a capital city?
107. #phpsc17 Do all countries have a capital city? Switzerland does not. The government is currently in Bern, but the city is not the capital
108. Commerce
109. #phpsc17 A product has one price
110. #phpsc17 A product has one price Price with tax, price without tax, sale price, list price, minimum price
111. @mheap A product has a price
112. @mheap A product has a price Products sold on auction sites do not have a price until it’s sold
113. #phpsc17 Currencies symbols uniquely identify a currency
114. #phpsc17 Currencies symbols uniquely identify a currency The peso and dollar sign $ is used by many countries
115. @mheap Currencies have a unicode symbol that represents them
116. @mheap Currencies have a unicode symbol that represents them The Swiss franc does not Until 2010, neither did the Indian rupee
117. #phpsc17 Currencies symbols uniquely identify a currency
118. #phpsc17 Currencies symbols uniquely identify a currency The peso and dollar sign $ is used by many countries
119. Audience Participation
120. @mheap Locale doesn’t matter for currency
121. @mheap Locale doesn’t matter for currency 1.000
122. #phpsc17 The currency symbol always precedes the amount e.g. £1.50
123. #phpsc17 The currency symbol always precedes the amount e.g. £1.50 1.50€ in Spain
124. @mheap The currency symbol always precedes or follows the amount
125. @mheap The currency symbol always precedes or follows the amount 1€50 in France
126. Computers
127. The network is reliable. Latency is zero. Bandwidth is infinite. The network is secure. Topology doesn't change. There is one administrator. Transport cost is zero. The network is homogeneous.
128. (?:(?:\r\n)?[ \t])*(?:(?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?: [^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000- \031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\ ["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*|(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?: \r\n)?[ \t])*)*\<(?:(?:\r\n)?[ \t])*(?:@(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?: [^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*(?:,@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?: \r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\ [([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*)*:(?:(?:\r\n)?[ \t])*)?(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?: \r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)? [ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?: (?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*\>(?:(?:\r\n)?[ \t])*)|(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\ [\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)*:(?:(?:\r\n)?[ \t])*(?:(?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?: (?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)? [ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\". \[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*|(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\ [\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)*\<(?:(?:\r\n)?[ \t])*(?:@(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\] (?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*(?:,@(?:(?:\r\n)?[ \t])*(?: [^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?: \r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*)*:(?:(?:\r\n)?[ \t])*)?(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\ [\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?: \r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?: \r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*\>(?:(?:\r\n)?[ \t])*)(?:,\s*(?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)? [ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\ \".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)? [ \t])*))*|(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)*\<(?:(?:\r\n)?[ \t])*(?:@(?:[^()<>@,;:\ \".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z| (?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*(?:,@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\ \.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*))*)*:(?:(?:\r\n)?[ \t])*)? (?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000- \031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+| \Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\\.)*\](?:(?:\r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|\[([^\[\]\r\\]|\ \.)*\](?:(?:\r\n)?[ \t])*))*\>(?:(?:\r\n)?[ \t])*))*)?;\s*)
129. In summary
130. In summary Computers are hard
131. In summary
132. In summary Humans make assumptions
133. In summary
134. In summary Things are never as simple as they seem
135. @mheap https://joind.in/21483