set
(and the function
set)
in the
Python
Standard Library
The value of each key in the
dictionary
is
None
(line
62).
The output is 44 lines, containly only one copy of Grover Cleveland:
George Washington John Adams Thomas Jefferson James Madison James Monroe John Quincy Adams Andrew Jackson Martin Van Buren William Henry Harrison John Tyler James K. Polk Zachary Taylor Millard Fillmore Franklin Pierce James Buchanan Abrajam Lincoln Andrew Johnson Ulysses S. Grant Rutherford B. Hayes James A. Garfield Chester A. Arthur Grover Cleveland Benjamin Harrison William McKinley Theodore Roosevelt William Howard Taft Woodrow Wilson Warren G. Harding Calvin Coolidge Herbert Hoover Franklin D. Roosevelt Harry S. Truman Dwight D. Eisenhower John F. Kennedy Lyndon B. Johnson Richard Nixon Gerald Ford Jimmy Carter Ronald Reagan George H. W. Bush Bill Clinton George W. Bush Barack Obama Donald Trump
dictionaryOfPresidents = {} #Start with an empty dictionary.
for president in listOfPresidents[1:]: #Skip the dummy item.
if president not in dictionaryOfPresidents:
dictionaryOfPresidents[president] = None #Seeing him for the first time.
print(president)
to the following.
I slapped on the line numbers
to make it easy for you to count the lines of output.
setOfPresidents = set(listOfPresidents[1:]) #Do not include the dummy item. for i, president in enumerate(setOfPresidents, start = 1): print(f"{i:2} {president}")
The order has not been preserved, but the extra copy of Grover Cleveland has been removed. There are only 44 lines of output.
1 Andrew Johnson 2 George Washington 3 Ulysses S. Grant 4 John Quincy Adams 5 Chester A. Arthur 6 Franklin D. Roosevelt 7 James K. Polk 8 Grover Cleveland 9 Thomas Jefferson 10 Dwight D. Eisenhower 11 Abraham Lincoln 12 Andrew Jackson 13 Zachary Taylor 14 John Tyler 15 Theodore Roosevelt 16 Franklin Pierce 17 Rutherford B. Hayes 18 James Monroe 19 William Howard Taft 20 Jimmy Carter 21 James Madison 22 Gerald Ford 23 George H. W. Bush 24 Warren G. Harding 25 Lyndon B. Johnson 26 John F. Kennedy 27 William Henry Harrison 28 James Buchanan 29 Martin Van Buren 30 Herbert Hoover 31 Woodrow Wilson 32 James A. Garfield 33 Harry S. Truman 34 John Adams 35 Bill Clinton 36 Benjamin Harrison 37 Richard Nixon 38 Millard Fillmore 39 George W. Bush 40 Calvin Coolidge 41 Ronald Reagan 42 Barack Obama 43 Donald Trump 44 William McKinley
ps
lists all the current
processes
(running programs), one per line.
The command line option -A
stands for “all”;
the
-o comm
stands for “output the name of the command”.
Here are the first ten and last ten lines of the output of the
ps
program.
The first line of output,
COMM,
is a header line.
Many of the other lines are
pathnames,
separated by slashes into components.
Some lines (e.g.,
login,
-bash)
appear more than once.
We follow the
ps
immediately with an
echo $?
to demonstrate that the
ps
produced exit status 0.
For the
$?,
see
“status of the most recently executed foreground pipeline”
in the documentation for the Bourne Again shell
bash.
A similar command in the Microsoft Windows command prompt is
tasklist.
ps -A -o comm COMM /sbin/launchd /usr/libexec/UserEventAgent /usr/sbin/syslogd /System/Library/PrivateFrameworks/Uninstall.framework/Resources/uninstalld /usr/libexec/kextd /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/FSEvent /System/Library/PrivateFrameworks/MediaRemote.framework/Support/mediaremoted /System/Library/CoreServices/appleeventsd /usr/libexec/configd etc. /usr/sbin/spindump /System/Library/Frameworks/CoreServices.framework/Frameworks/Metadata.framework /System/Library/Frameworks/QuickLook.framework/Resources/quicklookd.app/Content /Library/Frameworks/Python.framework/Versions/3.6/Resources/Python.app/Contents login -bash ssh login -bash ps echo $? 0
The
os.popen
function in
line
10
of
ps.py
launches the
ps
program.
The Python script is therefore called the
parent processs
and the
ps
program is called its
child process.
ps
is at line number 181 in the following output.
(Python itself is line 41, and my
bash
shell is line 1.)
os.popen
also opens a
pipe
that carries data
(in this case, lines of text)
from the child process to the parent process.
1 -bash 2 AirPlayUIAgent 3 AirPlayXPCHelper 4 AlertNotificationService 5 AppleCameraAssistant 6 AppleIDAuthAgent 7 AppleSpell 8 AssetCache 9 AssetCacheLocatorService 10 CVMServer 11 CalNCService 12 CalendarAgent 13 CallHistoryPluginHelper 14 CallHistorySyncHelper 15 CloudKeychainProxy 16 CommCenter 17 ContactsAccountsService 18 ContactsAgent 19 ContainerMetadataExtractor 20 CoreLocationAgent 21 CrashReporterSupportHelper 22 DataDetectorsSourceAccess 23 Dock 24 DuetHeuristic-BM 25 EscrowSecurityAlert 26 Finder 27 FolderActionsDispatcher 28 Google Chrome 29 Google Chrome Helper 30 IDLE 31 IMDPersistenceAgent 32 IMRemoteURLConnectionAgent 33 KernelEventAgent 34 KeychainSyncingOverIDSProxy 35 KiesAgent 36 LaterAgent 37 LookupViewService 38 NotificationCenter 39 PAH_Extension 40 ProtectedCloudKeySyncing 41 Python 42 SafariCloudHistoryPushAgent 43 ScopedBookmarkAgent 44 Siri 45 SocialPushAgent 46 Spotlight 47 SubmitDiagInfo 48 SystemUIServer 49 TISwitcher 50 TMCacheDelete 51 Terminal 52 USBAgent 53 UserEventAgent 54 VDCAssistant 55 VTDecoderXPCService 56 ViewBridgeAuxiliary 57 WiFiAgent 58 WiFiProxy 59 WiFiVelocityAgent 60 WindowServer 61 WirelessRadioManagerd 62 accountsd 63 airportd 64 akd 65 amfid 66 appleeventsd 67 apsd 68 askpermissiond 69 aslmanager 70 assistant_service 71 assistantd 72 authd 73 autofsd 74 awdd 75 bird 76 blued 77 callservicesd 78 captiveagent 79 cdpd 80 cfprefsd 81 cloudd 82 cloudpaird 83 cloudphotosd 84 com.apple.AmbientDisplayAgent 85 com.apple.CloudPhotosConfiguration 86 com.apple.CodeSigningHelper 87 com.apple.CommerceKit.TransactionService 88 com.apple.DictionaryServiceHelper 89 com.apple.GSSCred 90 com.apple.PerformanceAnalysis.animationperfd 91 com.apple.PhotoIngestService 92 com.apple.audio.DriverHelper 93 com.apple.audio.SandboxHelper 94 com.apple.ctkpcscd 95 com.apple.dock.extra 96 com.apple.geod 97 com.apple.iTunesLibraryService 98 com.apple.ifdreader 99 com.apple.notificationcenterui.WeatherSummary 100 com.apple.photomoments 101 com.apple.quicklook.ThumbnailsAgent 102 com.apple.sbd 103 com.apple.speech.speechsynthesisd 104 configd 105 coreaudiod 106 coreauthd 107 corebrightnessd 108 coreduetd 109 coreservicesd 110 corestoraged 111 coresymbolicationd 112 crashpad_handler 113 ctkahp 114 ctkd 115 deleted 116 diagnostics_agent 117 diskarbitrationd 118 diskspaced 119 displaypolicyd 120 distnoted 121 filecoordinationd 122 findmydeviced 123 fmfd 124 followupd 125 fontd 126 fseventsd 127 fuspredownloader 128 gamecontrollerd 129 gamed 130 geodMachServiceBridge 131 hidd 132 iTunesHelper 133 icdd 134 iconservicesagent 135 iconservicesd 136 identityservicesd 137 imagent 138 imklaunchagent 139 installd 140 installerdiagd 141 kextd 142 keyboardservicesd 143 launchd 144 launchservicesd 145 locationd 146 logd 147 login 148 logind 149 loginwindow 150 lsd 151 mDNSResponder 152 mDNSResponderHelper 153 mapspushd 154 mdflagwriter 155 mds 156 mds_stores 157 mdworker 158 media-indexer 159 mediaremoteagent 160 mediaremoted 161 mobileassetd 162 nbagent 163 nbstated 164 nehelper 165 netbiosd 166 networkserviceproxy 167 notifyd 168 nsurlsessiond 169 nsurlstoraged 170 ntpd 171 opendirectoryd 172 parsecd 173 passd 174 pboard 175 pbs 176 periodic-wrapper 177 photoanalysisd 178 photolibraryd 179 pkd 180 powerd 181 ps 182 quicklookd 183 revisiond 184 sandboxd 185 secd 186 secinitd 187 securityd 188 securityd_service 189 sharedfilelistd 190 sharingd 191 soagent 192 softwareupdate_notify_agent 193 softwareupdated 194 spindump 195 spindump_agent 196 ssh 197 ssh-agent 198 storeaccountd 199 storeassetd 200 storedownloadd 201 storeinstalld 202 storelegacy 203 suggestd 204 suhelperd 205 symptomsd 206 syslogd 207 sysmond 208 syspolicyd 209 system_installd 210 systemsoundserverd 211 systemstats 212 taskgated 213 tccd 214 thermald 215 trustd 216 uninstalld 217 usbd 218 usbmuxd 219 useractivityd 220 usernoted 221 videosubscriptionsd 222 warmd 223 watchdogd 224 wdhelper 225 wifivelocityd 226 wirelessproxd
ps
produces exit status 1 if you misspell
comm.
ps -A -o komm ps: komm: keyword not found ps: no valid keywords; valid keywords: %cpu %mem acflag acflg args blocked caught comm command cpu cputime etime f flags gid group ignored inblk inblock jobc ktrace ktracep lim login logname lstart majflt minflt msgrcv msgsnd ni nice nivcsw nsignals nsigs nswap nvcsw nwchan oublk oublock p_ru paddr pagein pcpu pending pgid pid pmem ppid pri pstime putime re rgid rgroup rss ruid ruser sess sig sigmask sl start stat state stime svgid svuid tdev time tpgid tsess tsiz tt tty ucomm uid upr user usrpri utime vsize vsz wchan wq wqb wql wqr xstat echo $? 1If the child process’s exit status is not zero, the
close
method returns the child process’s exit status
multiplied by 256 on macOS and Linux.
(See
“left-shifted by one byte”
in the
os.popen
documentation.)
There is no multiplication on Microsoft Windows.
(In Microsoft Windows, the exit status is called the
%errorlevel%.)
infile = os.popen("ps -A -o komm") #Create a child process and a pipe.
"ps -A -o komm" produced exit status 1.
lines = [line.rsplit("/", maxsplit = 1)[-1] for line in lines]to
lines = [os.path.basename(line) for line in lines]to eliminate an operating system dependency (macOS and Linux
"/"
vs.
Microsoft Windows
"\\").
-bash,
because the ASCII code of a dash
(hexadecimal 2D)
is less than the ASCII code of any letter.
See the ASCII code numbers in
For.
(For the dash,
see the “hyphen” in
login,
far beyond the scope of this course.)
Let’s ignore a leading dash when we sort the names.
Change
lines = sorted(lines)in line 23 to
def score(line): "Strip a leading dash from line." return line.lstrip("-") lines = sorted(lines, key = score)or to
lines = sorted(lines, key = lambda line: line.lstrip("-"))
73 autofsd 74 awdd 75 -bash 76 bird 77 blued
infile = os.popen("ps -A -o comm")
for i, line in enumerate(sorted(
[os.path.basename(line) for line in set(infile.read().splitlines()[1:])],
key = lambda line: line.lstrip("-")), start = 1):
print(f"{i:3} {line}")
import sys import os import csv infile = os.popen("tasklist /fo csv /nh") #format CSV, no header lines = csv.reader(infile) lines = sorted(set([line[0] for line in lines])) status = infile.close() if status != None: print("status =", status) #this is the Microsoft %errorlevel% sys.exit(1) for i, line in enumerate(lines, start = 1): print(f"{i:3} {level}") sys.exit(0)