CODEWITHSUNDEEP
pythonsympy
Sohail Si
Sohail Si

Reputation: 2976

Sympy substitute does not work with real variables

In Sympy, when a symbol is real (real=True), it is not substituted using subs(). Even when real is false, it doesn't work. Whenever real is specified in symbols() call, the substitution subs() does not work. Otherwise, it works:

import sympy

X = sympy.symbols("X")
print('i', X.subs(  {'X': 1.01}  )) # works

X = sympy.symbols("X", real=True) # `real=` Seems to be the cause of the problem
print('ii', X.subs(  {'X': 2.02}  ))

X = sympy.symbols("X", real=False) # even this
print('iii', X.subs(  {'X': 3.03}  ))

output:

i 1.01000000000000
ii X
iii X

For some reason, I need to specify the assumption real=True.

My sympy.__version__ is : 1.10.1

Upvotes: 1

Views: 504

Answers (1)

Davide_sd
Davide_sd

Reputation: 13150

That behavior is to be expected: when you call .subs({'X': 3.03}), the 'X' is going to be "simpified" ie, it is going to be converted to a generic new symbol (with no assumptions).

In your case:

X = sympy.symbols("X", real=False) # even this
print('iii', X.subs(  {'X': 3.03}  ))
# iii X

that's because X is a symbol with assumptions, and 'X' will be converted to a symbol with no assumptions: they are different symbols, even though they share the name!

To understand it better, you can do something like this:

x1 = sympy.symbols("x")
x2 = sympy.symbols("x", real=True)
print(x1.equals(x2))
# out: False
print(x1 == x2)
# out: False

When you call subs you should be as specific as possible:

X = sympy.symbols("X", real=False) # even this
print('iii', X.subs(  {X: 3.03}  ))
# out: iii 3.03000000000000

Keep in mind that subs is meant to work on larger expressions, composed by one or multiple symbols.

Upvotes: 4

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