Python Cheat Sheet

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Notes for
PYTHON PROGRAMMING

Sequence Containers Indexing
Base Types
Python 3 Cheat Sheet©2012-2015 - Laurent Pointal
License Creative Commons Attribution 4
Latest version on :
https://perso.limsi.fr/pointal/python:memento
0783 -192int
9.23 -1.7e-60.0float
True Falsebool
"OnenTwo"
'I'm'
str
"""XtYtZ
1t2t3"""
×10
-6
escaped tab
escaped new line
Multiline string:
Container Types
list [1,5,9] ["x",11,8.9] ["mot"] []
tuple (1,5,9) 11,"y",7.4 ("mot",) ()
dict
{1:"one",3:"three",2:"two",3.14:"π"}
{"key":"value"}
set
{}
{1,9,3,0}
◾ ordered sequences, fast index access, repeatable values
set()
◾ key containers, no a priori order, fast key access, each key is unique
{"key1","key2"}
Non modifiable values (immutables)
Variables assignment
x=1.2+8+sin(y)
y,z,r=9.2,-7.6,0
a…zA…Z_ followed by a…zA…Z_0…9
◽ diacritics allowed but should be avoided
◽ language keywords forbidden
◽ lower/UPPER case discrimination
☝ expression with only comas →tuple
dictionary
collection
integer, float, boolean, string, bytes
Identifiers
☺ a toto x7 y_max BigOne
☹ 8y and for
x+=3
x-=2
increment ⇔ x=x+3
decrement ⇔ x=x-2
Conversions
for lists, tuples, strings, bytes…
int("15") → 15
int("3f",16) → 63 can specify integer number base in 2nd
parameter
int(15.56) → 15 truncate decimal part
float("-11.24e8") → -1124000000.0
round(15.56,1)→ 15.6 rounding to 1 decimal (0 decimal → integer number)
bool(x) False for null x, empty container x , None or False x ; True for other x
str(x)→ "…" representation string of x for display (cf. formatting on the back)
chr(64)→'@' ord('@')→64 code ↔ char
repr(x)→ "…" literal representation string of x
bytes([72,9,64]) → b'Ht@'
list("abc") → ['a','b','c']
dict([(3,"three"),(1,"one")]) → {1:'one',3:'three'}
set(["one","two"]) → {'one','two'}
separator str and sequence of str → assembled str
':'.join(['toto','12','pswd']) → 'toto:12:pswd'
str splitted on whitespaces → list of str
"words with spaces".split() → ['words','with','spaces']
str splitted on separator str → list of str
"1,4,8,2".split(",") → ['1','4','8','2']
sequence of one type → list of another type (via list comprehension)
[int(x) for x in ('1','29','-3')] → [1,29,-3]
type(expression)
lst=[10, 20, 30, 40, 50]
lst[1]→20
lst[-2]→40
0 1 2 3 4
-5 -4 -3 -1-2 Individual access to items via lst[index]
positive index
negative index
0 1 2 3 54
-5 -4 -3 -1-2negative slice
positive slice
Access to sub-sequences via lst[start slice:end slice:step]
len(lst)→5
lst[1:3]→[20,30]
lst[::2]→[10,30,50]
lst[-3:-1]→[30,40]
lst[:3]→[10,20,30]lst[:-1]→[10,20,30,40]
lst[3:]→[40,50]lst[1:-1]→[20,30,40]
lst[:]→[10,20,30,40,50]
Missing slice indication → from start / up to end.
On mutable sequences (list), remove with del lst[3:5] and modify with assignment lst[1:4]=[15,25]
Conditional Statement
if age<=18:
state="Kid"
elif age>65:
state="Retired"
else:
state="Active"
Boolean Logic Statements Blocks
parent statement:
statement block 1…
⁝
parent statement:
statement block2…
⁝
next statement after block 1
indentation !
Comparisons : < > <= >= == !=
≠=≥≤
a and b
a or b
not a
logical and
logical or
logical not
one or other
or both
both simulta-
-neously
if logical condition:
statements block
statement block executed only
if a condition is true
Can go with several elif, elif... and only one
final else. Only the block of first true
condition is executed.
lst[-1]→50
lst[0]→10
⇒ last one
⇒ first one
x=None « undefined » constant value
Maths
Operators: + - * / // % **
× ÷
integer ÷ ÷ remainder
ab
from math import sin,pi…
sin(pi/4)→0.707…
cos(2*pi/3)→-0.4999…
sqrt(81)→9.0 √
log(e**2)→2.0
ceil(12.5)→13
floor(12.5)→12
escaped '
☝ floating numbers… approximated values angles in radians
(1+5.3)*2→12.6
abs(-3.2)→3.2
round(3.57,1)→3.6
pow(4,3)→64.0
for variables, functions,
modules, classes… names
Mémento v2.0.6
str (ordered sequences of chars / bytes)
(key/value associations)
☝ pitfall : and and or return value of a or
of b (under shortcut evaluation).
⇒ ensure that a and b are booleans.
(boolean results)
a=b=c=0 assignment to same value
multiple assignments
a,b=b,a values swap
a,*b=seq
*a,b=seq
unpacking of sequence in
item and list
bytes
bytes
b"totoxfe775"
hexadecimal octal
0b010 0xF30o642
binary octal hexa
""
empty
dict(a=3,b=4,k="v")
Items count
☝ keys=hashable values (base types, immutables…)
True
False True and False constants ☝ configure editor to insert 4 spaces in
place of an indentation tab.
lst[::-1]→[50,40,30,20,10]
lst[::-2]→[50,30,10]
1) evaluation of right side expression value
2) assignment in order with left side names
=
☝ assignment ⇔ binding of a name with a value
☝ immutables
On mutable sequences (list), remove with
del lst[3] and modify with assignment
lst[4]=25
del x remove name x
b""
@ → matrix × python3.5+numpy
☝ index from 0
(here from 0 to 4)
frozenset immutable set
Priority (…)
☝ usual order of operations
modules math, statistics, random,
decimal, fractions, numpy, etc. (cf. doc)
Modules/Names Imports
from monmod import nom1,nom2 as fct
module truc⇔file truc.py
→direct access to names, renaming with as
import monmod →access via monmod.nom1 …
☝ modules and packages searched in python path (cf sys.path)
?
yes
no
shallow copy of sequence
?
yes no
and
*=
/=
%=
…
☝ with a var x:
if bool(x)==True: ⇔ if x:
if bool(x)==False: ⇔ if not x:
raise ExcClass(…)
Signaling an error:
Errors processing:
try:
normal procesising block
except Exception as e:
error processing block
normal
processing
error
processing
error
processing
raiseraise X()
zero
☝ finally block for final processing
in all cases.
Exceptions on Errors

"modele{} {} {}".format(x,y,r)
"{selection:formatting!conversion}"
◽ Selection :
2
nom
0.nom
4[key]
0[2]
str
Displayprint("v=",3,"cm :",x,",",y+4)
print options:
◽ sep=" " items separator, default space
◽ end="n" end of print, default new line
◽ file=sys.stdout print to file, default standard output
items to display : literal values, variables, expressions
loop on dict/set ⇔ loop on keys sequences
use slices to loop on a subset of a sequence
Conditional Loop Statementstatements block executed as long as
condition is true
while logical condition:
statements block
s = 0
i = 1
while i <= 100:
s = s + i**2
i = i + 1
print("sum:",s)
initializations before the loop
condition with a least one variable value (here i)
s= ∑
i=1
i=100
i2
☝ make condition variable change !
statements block executed for each
item of a container or iterator
for var in sequence:
statements block
s = "Some text"
cnt = 0
for c in s:
if c == "e":
cnt = cnt + 1
print("found",cnt,"'e'")
Go over sequence's values
Algo: count
number of e
in the string.
Go over sequence's index
◽ modify item at index
◽ access items around index (before / after)
lst = [11,18,9,12,23,4,17]
lost = []
for idx in range(len(lst)):
val = lst[idx]
if val > 15:
lost.append(val)
lst[idx] = 15
print("modif:",lst,"-lost:",lost)
Algo: limit values greater
than 15, memorizing
of lost values.
☝bewareofinfiniteloops!
initializations before the loop
loop variable, assignment managed by for statement
values to formatformating directives
Integer Sequences
Files
s = input("Instructions:")
☝ input always returns a string, convert it to required type
(cf. boxed Conversions on the other side).
range(5)→ 0 1 2 3 4 range(2,12,3)→ 2 5 8 11
range(3,8)→ 3 4 5 6 7 range(20,5,-5)→ 20 15 10
range(len(seq))→ sequence of index of values in seq
☝ range provides an immutable sequence of int constructed as needed
range([start,] end [,step])
f = open("file.txt","w",encoding="utf8")
storing data on disk, and reading it back
opening mode
◽ 'r' read
◽ 'w' write
◽ 'a' append
◽ …'+' 'x' 'b' 't'
encoding of
chars for text
files:
utf8 ascii
latin1 …
name of file
on disk
(+path…)
file variable
for operations
f.write("coucou")
f.writelines(list of lines)
writing reading
f.read([n]) → next chars
if n not specified, read up to end !
f.readlines([n]) → list of next lines
f.readline() → next line
with open(…) as f:
for line in f :
# processing ofline
cf. modules os, os.path and pathlib
f.close() ☝ dont forget to close the file after use !
Very common: opening with a guarded block
(automatic closing) and reading loop on lines
of a text file:
Function Definition
def fct(x,y,z):
"""documentation"""
# statements block, res computation, etc.
return res
function name (identifier)
result value of the call, if no computed
result to return: return None
☝ parameters and all
variables of this block exist only in the block and during the function
call (think of a “black box”)
named parameters
Function Callr = fct(3,i+2,2*i)
☝ read empty string if end of file
len(c)→ items count
min(c) max(c) sum(c)
sorted(c)→ list sorted copy
val in c → boolean, membership operator in (absence not in)
enumerate(c)→ iterator on (index, value)
zip(c1,c2…)→ iterator on tuples containing ci
items at same index
all(c)→ True if all c items evaluated to true, else False
any(c)→ True if at least one item of c evaluated true, else False
☝ modify original list
lst.append(val) add item at end
lst.extend(seq) add sequence of items at end
lst.insert(idx,val) insert item at index
lst.remove(val) remove first item with value val
lst.pop([idx])→value remove & return item at index idx (default last)
lst.sort() lst.reverse() sort / reverse liste in place
"{:+2.3f}".format(45.72793)
→'+45.728'
"{1:>10s}".format(8,"toto")
→' toto'
"{x!r}".format(x="I'm")
→'"I'm"'
☝ start default 0, end not included in sequence, step signed, default 1
◽ Conversion : s (readable text) or r (literal representation)
< > ^ = 0 at start for filling with 0
integer: b binary, c char, d decimal (default), o octal, x or X hexa…
float: e or E exponential, f or F fixed point, g or G appropriate (default),
string: s … % percent
◽ Formatting :
fill char alignment sign mini width.precision~maxwidth type
+ - space
Operations on Dictionaries Operations on Sets
Operators:
| → union (vertical bar char)
& → intersection
- ^ → difference/symmetric diff.
< <= > >= → inclusion relations
Operators also exist as methods.
d.update(d2) update/add
associations
Note: For dictionaries and sets, these
operations use keys.
Specific to ordered sequences containers (lists, tuples, strings, bytes…)
reversed(c)→ inversed iterator c*5→ duplicate c+c2→ concatenate
c.index(val)→ position c.count(val)→ events count
Operations on Lists
d[key]=value
d[key]→ value
d.keys()
d.values()
d.items()
d.clear()
del d[key]
→iterable views on
keys/values/associations
Examples
d.pop(key[,default])→ value
d.popitem()→ (key,value)
d.get(key[,default])→ value
d.setdefault(key[,default])→value
s.update(s2) s.copy()
s.add(key) s.remove(key)
s.discard(key) s.clear()
s.pop()
Loop Control
Go simultaneously over sequence's index and values:
for idx,val in enumerate(lst):
☝goodhabit :don'tmodifyloopvariable
Advanced: def fct(x,y,z,*args,a=3,b=5,**kwargs):
*args variable positional arguments (→tuple), default values,
**kwargs variable named arguments (→dict)
one argument per
parameter
storage/use of
returned value
Algo:
f.flush() write cache
f.tell()→position
reading/writing progress sequentially in the file, modifiable with:
f.seek(position[,origin])
f.truncate([size]) resize
Advanced:
*sequence
**dict
s.startswith(prefix[,start[,end]])
s.endswith(suffix[,start[,end]]) s.strip([chars])
s.count(sub[,start[,end]]) s.partition(sep)→ (before,sep,after)
s.index(sub[,start[,end]]) s.find(sub[,start[,end]])
s.is…() tests on chars categories (ex. s.isalpha())
s.upper() s.lower() s.title() s.swapcase()
s.casefold() s.capitalize() s.center([width,fill])
s.ljust([width,fill]) s.rjust([width,fill]) s.zfill([width])
s.encode(encoding) s.split([sep]) s.join(seq)
?
yes
no
next
finish
…
Input
import copy
copy.copy(c)→ shallow copy of container
copy.deepcopy(c)→ deep copy of container
☝ this is the use of function
name with parentheses
which does the call
fct()
fct
fct
☝ text mode t by default (read/write str), possible binary
mode b (read/write bytes). Convert from/to required type !
break immediate exit
continue next iteration
☝ else block for normal
loop exit.
Iterative Loop Statement
Operations on Strings
Formatting
Generic Operations on Containers

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