A Cheatsheet for Fortran 2008 Syntax: Comparison with Python 3

Wu Sun

Last updated: 2022-02-11

This is a simple cheatsheet to aid scientific programmers who work at the interfaces of both languages. It is by no means an exhaustive list. (I have yet to explore some advanced features in the Fortran 2008 and newer standards.)

Make Fortran Modern Again!

This work is licensed under the Creative Commons Attribution 4.0 International License.

	Fortran 2008	Python 3
Top-level constructs
the main program	program my_program ... end program	Not required. Use __main__() if needed.
modules	module my_module ... end module my_module	A source file is a module.
subroutines	subroutine my_subroutine ... end subroutine my_subroutine	Functions that have side effects.
functions	function f(x) result(res) res = ... end function f	def my_function(x): ... return res
submodules	module main_module ... contains <submodule statements> end module main_module	Use folders, __init__.py, and import statement to manage submodules.
import statement	use my_module use my_module, only : fun1, var1 use my_module, only : new_var => var	from my_module import * from my_module import fun1, var1 from my_module import var as new_var
call subroutines and functions	call my_subroutine(args) my_function(args)	my_function(args)
abort a program	stop	exit()
inline comments	! This is a comment	# This is a comment
line continuation	&	\
include external source files	include 'source_file_name'	Not supported. Use module import.
Control flow patterns
if construct	if <logical expr> then ... else if <logical expr> then ... else ... end if	if <logical expr>: ... elif <logical expr>: ... else: ...
case construct	select case <expr> case <value> ... case <value> ... case default ... end select	Structural pattern matching, supported since Python 3.10: match <expr>: case <pattern>: ... case <pattern>: ... case _: ...
do construct	do start_value, end_value, step ... end do	for i in range(start, end, step): ...
do while construct	do while <logical expr> ... end do	while <logical expr>: ...
break from a loop	exit	break
leave this iteration and continue to the next iteration	cycle	continue
Data types
declaration	integer(kind=kind_number) :: n = 0 real(kind=kind_number) :: x = 0.	n = 0 x = 0.
named constants	integer, parameter :: answer = 42 real(8), parameter :: pi = 4d0 * atan(1d0)	Not supported in Python. Names are bindings.
complex number	complex :: z = (1., -1.)	z = 1 - 1j
string	character(len=10) :: str_fixed_length character(len=:), allocatable :: str_var_length	string = "this is a string"
pointer	real, pointer :: p real, target :: r p => r	No built-in support. May invoke from Python C API. (Why would you need it in Python anyway?)
boolean	.true. .false.	True False
logical operators	.not. .and. .or. .eqv. .neqv.	not and or Other logical operators do not have built-in support.
equal to	==, .eq.	==
not equal to	/=, .ne.	!= Note: <> is deprecated in Python 3.
greater than	>, .gt.	>
less than	<, .lt.	<
greater than or equal to	>=, .ge.	>=
less than or equal to	<=, .ge.	<=
array declaration	real(8), dimension(3) :: a = [1., 2., 3.]	import numpy as np a = np.array([1., 2., 3.])
string array declaration	character(len=20), dimension(3, 4) :: char_arr	char_arr = np.chararray((3, 4), itemsize=20)
elementwise array operations	a op b op can be +, -, *, /, **, =, ==, etc. This is supported since the Fortran 90 standard.	Supported through numpy.ndarray type.
first element	a(1)	a[0]
slicing	a(1:5) This slice includes a(5).	a[0:5] This slice does not include a[5].
slicing with steps	a(1:100:2)	a[0:100:2]
size	size(a)	a.size
shape	shape(a)	a.shape
shape along a dimension	size(a, dim)	a.shape[dim]
Type conversion
to integer by truncation	int()	int()
to integer by rounding	nint()	int(round())
integer to float	real(a[, kind])	float()
complex to real	real(z[, kind])	z.real
to complex	cmplx(x [, y [, kind]])	complex()
to boolean	logical()	bool()
Derived data types
definition	type Point real(8) :: x, y end type Point	class Point(object): def __init__(self, x, y): self.x = x self.y = y
instantiation	type(Point) :: point1 = Point(-1., 1.)	point1 = Point(-1., 1.)
get attributes	point1%x point1%y	point1.x point1.y
array of derived type	type(Point), dimension(:), allocatable :: point_arr	No built-in support. You may need to subtype the NumPy array. (Why not use a structured array instead of an array of structure?)
type bound procedures (aka class method)	Assume that Circle has a type bound procedure (subroutine) print_area. type(Circle) :: c call c%print_area	Assume that Circle has a class method print_area(). c = Circle() c.print_area()
Built-in mathematical functions
functions with the same names	abs(), cos(), cosh(), exp(), floor(), log(), log10(), max(), min(), sin(), sinh(), sqrt(), sum(), tan(), tanh()	Have the same name in NumPy.
functions with different names	acos() aimag() asin() atan() atan2(x, y) ceiling() conjg(z) modulo() call random_number()	np.arccos() z.imag np.arcsin() np.arctan() np.arctan2(x, y) np.ceil() np.conjugate(z), z.conjugate() np.mod(), % np.random.random_sample()
Built-in string functions
string length	len()	len()
string to ASCII code	iachar()	ord()
ASCII code to string	achar()	chr()
string slicing	Same as 1D array slicing.	Same as 1D array slicing.
find the position of a substring	index(string, substring)	string.index(substring)
string concatenation	"hello" // "world"	"hello" + "world" "hello" "world" ("hello" "world") The operator may be omitted.
Array constructs
where construct	where a > 0 b = 0 elsewhere b = 1 end where	For NumPy arrays, b[a > 0] = 0 b[a <= 0] = 1 For lists, use the higher order function filter() or list comprehension.
forall construct	real, dimension(10, 10) :: a = 0 int :: i, j ... forall (i = 1:10, j = 1:10, i <= j) a(i, j) = i + j end forall	import itertools import numpy as np a = np.zeros((10, 10)) for i, j in itertools.product(range(10), range(10)): if i <= j: a[i, j] = i + j Note: this is a simple example to reproduce the forall construct in Python. There are definitely other (and more Pythonic) ways of doing the same thing.
Other built-in subroutines
CPU time	call cpu_time()	import time time.time()
command line arguments	call command_argument_count() call get_command() call get_command_argument()	For basic parsing, use sys.argv. For more advanced use, see the built-in argparse module.
Input/output
print	print fmt, <output list>	print()
read from the command prompt	read fmt, <input list>	input(prompt)
open a file	open(unit, file, ...)	f = open(file, mode='r', ...)
read from a file	read(unit, fmt, ...) <input list>	f.read(size) f.readline()
write to a file	write(unit, fmt, ...) <output list>	f.write()
close a file	close(unit, ...)	f.close()
file inquiry	inquire(unit, ...)	Use the file utilities provide by the os module.
backspace in a file	backspace(unit, ...)	f.seek(-1, 1) Note: this works only when the file is open as a binary file.
end of file (EOF)	endfile(unit, ...)	f.read() == '' However, this is generally not needed.
return to the start of a file	rewind(unit, ...)	f.seek(0)