Python

© 2019 Martin Bruchanov, bruchy@gmail.com

Command line parameters

• python options script.py – run script filename
• -V – print version
• -c 'code' – run code from command line
• python -m py_compile script.py – test syntax of script
• python3 -c 'import keyword; print(keyword.kwlist)' – keywords
• python -i, ipython – interactive mode

Create virtual environment

• python -m virtualenv /path/to/dir or python3 -m venv
• Make current shell to use it: source /path/to/dir/bin/activate
• Check if virtual env. is used: pip –version
• Quit virtual env.: deactivate

Expression statements

• import module – find and initialize module
• module.function() – use function of imported module
• from module import * – import all to local name space
• import module as name – rename imported module
• from module import name as othername
• break – exit while or for loop, skip associated else
• continue – perform next iteration of cycle
• quit([code=exit code]) – exit script and set return value
• raise Exception('Message') – create error during execution
• global name – reference global value
• exec("print('Ahoj')") – compile and exec code
• with expression [as variable]:
␣␣␣␣suite – block entry actions
• pass – do-nothing placeholder statement
• del name, del name[i], del name[i:j:k], del name.attibute – delete variables, items, keys, attributes
• assert expression [, message]
• exec codestring
• Generator expression:
  result expr. for loop var. in iterable if filter expr.
• s = 'Yes' if k == True else 'No' – ternary operator
• def noop(*args, **kws): return None – no-operation function

Classes

• class Name:
␣␣␣␣suite
• _private – underscored named object is private
• def __init__(self, ...):
  ␣␣␣␣self.data = [] – constructor
• class DerivedClass(BaseClass) – inheritance

Functions

• def function(param1, param2,...):
␣␣␣␣pass
• def func(arg,... arg=value, ... *arg, **arg):
  • arg – matched by name or position
  • arg=value – default value if arg is not passed
  • *arg – collect extra positional args as a new tuple
  • **arg – collect extra positional args as a new dictionary
• lambda args1 : expression – anonymous function maker
• map(lambda x: x.capitalize(), ['abc','def']) – example
• return [expression] – return from function
• yield expression – suspend function state and return, on next iteration restore prior state
• def gen(x):\n for i in range(x): yield i*i – returns gener. object

Variables

• variable = 12 – assign value
• type(variable) – return type of variable
• global name [,name] – global variable in local context
• Number formats:
  • 2006, 2006l, 2006L – decimal integer, long;
  • 0775, oct(0x1fd) – octal;
  • 0xBABE, hex(47806) – hexadecimal;
  • 0b101010, bin(42) – binary;
  • 3.14, 314e-2 – floating point;
  • 1+2j, 1.0+2.0J, complex(1,2) – complex number;
  • b'Ahoj' – sequence of 8-bit values;
• int(x), long(x), float(x), str(n) – type conversions
• int('GEEK', 21) – convert string number with given base
• c=1+2j; c.conjugate(), (1+2j).conjugate() – conjugate of complex number 1-2j
• abs(x) – absolute value of x
• round(x[,n]) – x rounded to n digits
• (10.5).as_integer_ratio() – returns tuple (21, 2)
• (255).bit_length() – number of digits of binary
• X, Y = Y, X – swap values of X and Y
• a,b,c = range(3) – read list values, a=0,b=1,c=2
• vars(), globals(), locals() – return dictionary of variables
• setattr(obj, 'b', c) is equivalent obj.b = c
• getattr(obj, 'a') is equivalent obj.a
• hasattr(obj, name) – True if name is object atribute

Constants

• False, True – boolean
• None – represents no value
• bool([X]) – returns boolean value of object X.

Operators

• or, and, not x – boolean operators
• | (or), ^ (xor), & (and), ~x (neg.) – binary operators
• X in Y, X not in Y – membership tests
• X is Y, X is not Y – same or different object
• <, <=, >, >=, <>, !=, == – comparisons
• *, /, //, % – multiply, divide, floor divide, remainder
• x << n, x >> n – bitwise shifts by n bits
• x**y, pow(x,y) – power x^y
• += &= -= |= *= ^= /= >>= %= <<= **= //=
• divmod(x,y) – return tuple (x/y, x%y)

Data types

Tuples

• t = (), t = tuple() – create empty tuple
• t = (1, 2, 3) – like list, but can't change their values
• t[1] – access second item, returns 2
• t.index(x [, i [, j]]) – return index of first occurrence of x
• t.count(x) – return number of item x

Lists

• l = [], l = list() – empty list
• l = [1, 2, 3] – one dimensional array
• [3] * 4 – repeated 4× to [3, 3, 3, 3]
• l[1] – returns 2, indexing: 1₀ 2₁ 3₂
• l[i:j] – slicing from index i to j-1
• l[i:] – slicing from index i to end of list
• l[i:j:k] – slicing with step k  ≈ pprox l[slice(i,j[,k])]
• l[-1] – last item (first from back)
• 0 in [1, 2, 3] – False, 1 in [1, 2, 3] True
• l = range(5) – create list [0, 1, 2, 3, 4]
• l = range(start, stop[, step]) – given range with step
• l = [x**2 for x in range(9)] – list from expression result
• l.index(item) – return index of item in list
• l.count(item) – total number of occurrences of item
• l = ["text", 12, 3, [1, 2]] – more types in one list
• l2d=[[1,2,3], [4,5,6], [7,8,9]] – two-dimensional list
• l2d[1][1] – returns 5
• list('abc') – returns list of chars ['a','b','c']
• len(l) – return length of list
• l.append(value) – add value to the list
• l.extend([4,5]), list[len(list):]=[4,5], list += [4,5] – append another list
• l.insert(i, x), list[i]=x – insert x at given index
• l[:0]=[x,y,z] – insert item at front of list
• l.remove(value) – remove first occurrence of value
• l.pop(i), l.pop() – return and remove value, without index last
• l.index(x [, i[, j]]) – index of first occur. of x, between i to j-1
• l.count(x) – return number of occurrence of object x
• l.sort(key=None, reverse=False) – sort list in-place
• l.reverse() – reverse list in-place
• sum(l) – return sum of numeric list
• sorted(set(l)) – sort and unique list via set
• sorted(unsorted_l, key=str.casefold) – sort case insensitive

Dictionaries

• h = {}, h = dict() – initialization of empty dictionary
• h = { "key1": "value", "key2": "another"} – definition
• h = dict(key1="value", key2="another") – different syntax
• h = dict([('key1', 'value'), ('key2', 'another']) – yet another
• h["key3"] = 333 – add another value
• h[(1+2j)] = 666 – also hashable object can be a key
• h = {c: ord(c) for c in 'spam'} – comprehension expression
• 'key' in h – returns True if key exist (was h.has_key("key"))
• h.keys(), h.values() – return list of keys, and values
• h.clear() – remove all items
• g = h.copy() – returns a shallow copy of h
• h.get(key [, default]) – if key is not found return default
• h.popitem() – removes and returns an (key, value) pair
• h.pop(k [, def]) – returns and removes k else return def
• del h['key1'] – remove entry for key key1
• h.fromkeys(seq [, value]) – new dictionary from keys in seq
• dict(zip(['a','b'], [1,2])) – join to {'a': 1, 'b': 2}
• for key, value in h.items(): – iterate dictionary
• g = {}; g.update(h), g = dict(h), g = dict(h.items()) – make copy

Sets

• A = set() – empty set      A={ ∅ }
• A = set('Ouagadougou') = A = set(['a','d','g','o','u','O']), unordered collection of unique and immutable objects
• A = {'a', 'd', 'g', 'o', 'u', 'O'} – set definition
• A = frozenset(range(−5, 5)) – immutable set of -5…4
• 'a' in A – returns True if value is presented      a ∈ A
• A - B, A.difference(B) – new set contains difference     A \ B
• A | B, A.union(B) – join two sets, no duplicates      A ⋃ B
• A & B, A.intersection(B) – same items in both sets      A ⋂ B
• A <= B, A.issubset(B) – returns True is A is subset of B      A ⊂ B
• A >= B, A.issuperset(B) – is A superset of B?      A ⊃ B
• A < B, A > B – true subset, superset      A ⊂ B, A ⊃ B
• A ^ B, A.symmetric_difference(B) – A ∆ B = (A ⋃ B) \ (A ⋂ B)
• A |= B, A.update(B) – adds items in B to A
• A.discard(X) – remove item if exist
• A.add(X), A.remove(X) – add, remove item from set
• A.clear() – remove all items
• A.pop() – remove and return arbitrary item
• len(A) – get number of items in A
• for x in A: – all iteration context
• B=A.copy(), B=set(A) – make copy of set

Strings

• s = "Hello", s = 'Hello' – definition, " and ' works same
• s[::-1] – reverse string (olleH)
• """This is multi-line block""" – collects into a single string
• s[1]='e' – indexing H₀ e₁ l₂ l₃ o₄
• s[i:j] – substring from i to j-1 (same indexing as lists)
• str(n) – convert number n to string
• 'Hello ' + 'World', "Hello" "World" – concatenation
• 'Hello' * 3 – repetition 3×
• Unicode ‘→’: "\u2192", "\U00002192", "\N{Rightwards Arrow}"
• Raw string: r"\n", R'\n' does not interpret escape sequences
• Unicode raw string: ur"\n", UR'\n'
• str(), bytes(), bytearray() – create string from object
• \xhh, \ooo, \0 – hex, octal, null byte
• chr(65), unichr(65), ord('A') – returns character, ASCII code
• eval(s) – convert and execute code given by string
• execfile(filename) – like eval, but for whole file

Output and formating

• print(*objects, sep=' ', end='\n', file=sys.stdout)
• '%s, %s, %.2f' % (13, 'txt', 22/7.0) – '13, txt, 3.14'
• '{0}, {1}, {2:.2f}'.format(13,'txt',22/7.0) – other def.
• "%(a)d %(b)s" % {"a":6, "b":"text"} – formating dictionary
• "{a} {b}".format(**{'a':1, 'b':2}) – formating dictionary
• "%*s" % (10, "text") – width given as parameter
• "%#x %#o" % (15,15) – prints number base prefixes
• "%+.*f" % (5, 22.0/7) – +3.14286, 5 digits after ‘.’
• %[(keyname)][flags][width][.precision]typecode
• Flags: -/+ left/right justify, 0/' ' zero/space fill
• String formating typecodes:
  • s – String (or any object, uses str())
  • r, – s, but uses repr(), not str()
  • c – Character (int or str)
  • d, i, u – Decimal (base 10 integer)
  • o – Octal (base 8 integer)
  • x, X – Hex (base 16 integer)
  • e, E – Floating-point exponent
  • f, F – Floating-point decimal
  • g, G – Floating-point e,f/E,f
  • %% – Literal '%'
• {fieldname!conversionflag:formatspec}
• [[fill]align][sign][#][0][width][,][.prec][typecode]
• vprint = print if VERBOSE else lambda *a, **k: None – verbose prints

String methods

• s.find/rfind(sub, [,s [,e]]) – index of first occur. of sub,
• s.index/rindex(sub [,s [,e]]) – ValueError if not found
• s.endswith/startswith(sub [,s [,e]]) – true if starts/ends
• s.count(sub, [,s [,e]]) – get number of substrings
• s.upper(), s.lower(), s.swapcase() – converts case
• s.split([sep [, maxsplit]) – return list of words
• sep.join(iterable) – concatenates with separator
• ' and '.join(['a', 'b', 'c']) – returns 'a and b and c'
• s.replace(old, new [, count]) – replace old by new
• s.splitlines(0/1) – split by '\n', 1 – keeps end char
• s.strip([chars]) – remove leading and trailing white spaces
• s.lstrip, s.rstrip – just from left or right side
• s.center/ljust/rjust(width [,fill]) – justify string
• s.capitalize() / s.title() – make first/all word(s) uppercase
• s.expandtabs(tabsize) – replaces tabs with spaces (default 8)
• isalnum, isalpha, isdecimal, isdigit, isidentifier, islower, isnumeric, isprintable, isspace, istitle, isupper – tests

Other built-in functions

• help(object), help('function') – display documentation
• max(iterable), min(iterable) – return max/min value
• reversed(iterable) – return a reverse iterator
• sorted(iterable, key=None, reverse=False) – return sorted
• sorted(lt, key=lambda x: x[1]) – sort list of tuples by 2^nd element
• enumerate(iterable, start=0) – return an enumerate object
• all(iter), any(iter) – True if all/any of elements are/is true.
• hash(obj) – return hash value of object
• iter(o [,sentinel]) – return an iterator object
• next(iterator [,default]) – return next item from iterator
• map(function, iterable, ...) – apply function on every item
• input([prompt]) – read line for stdin

Work with files

• file=open('data.txt'[, 'mode']) – open, mode: r,w,rb,w,r+,w+
• s = file.read([n]) – read file of n bytes into string s
• file.readline() – return line of file, empty at EOF
• file.readlines() – read entire file into a list of line strings
• for line in file: – process file line by line
• file.write(s) – write string s into file
• print >>file, "Output" – write string to file
• file.writeline(list) – write all strings in list to file
• file.close() – close to free resources
• file.tell() – return file position
• file.seek(offset [, whence]) – set file position
• file.flush() – flushes file's buffer
• file.truncate([size]) – truncate file to size bytes
• file.fileno() – get file descriptor integer
• file.closed, file.mode, file.name – return attributes
• with open('file.txt', 'r') as f: – block with file manipulations

Regular expressions (import re)

• ro=re.compile(pattern, flags=0) – create RegexObject ‘ro’
• Flags: re.DOTALL (S), re.IGNORECASE (I), re.LOCALE (L), re.MULTILINE (M), re.VERBOSE (X), re.UNICODE (U)
• re.match(pattern, string) – if match return MatchObject
• re.search(pattern, string) – match regex anywhere in string
• re.split(pattern,string) – split pattern
• re.findall(pattern, string) – return substrings as list
• re.finditer(pattern, string) – return matches as iterator
• re.sub(pattern, repl, string, count=0, flags=0) – return string with replaced pattern
• re.subn(…) – return tuple (string, num. of replacements)
• re.escape(string) – string with escaped regex's metacharacters
• RegexObject methods: ro.match, search, split, sub, subn, findall, finditer
• ro.flags, ro.pattern – used argument for reg. obj. creation
• ro.groups() – number of matched groups
• ro.group(n) – return n^th string matched by regex
• ro.start(), ro.end(), ro.span() – return starting, ending position or tuple

System specific functions and parameters

• sys.argv – CLI parameters, argv[0] name of script
• sys.stdin.readline() – read line from standard input
• subprocess.call(["ls", "-l"]) – execute system command
• out = subprocess.check_output(['uname', '-a']) – store output of command to variable
• filelist = subprocess.Popen("ls *", shell=True,
stdout=subprocess.PIPE).communicate()[0] – read data from pipe
• os.stat('/path/to/file.txt') – return POSIX stat file info
• os.environ.get('PATH') – get value of environment variable PATH