title: Unexpected result of a substitution inside a function argument
body: <pre><code>In [4]: from sympy import pi, sin, solve, symbols
     ...: t, w0 = symbols('t omega_0')
     ...: wb = 6*w0
     ...: f = sin(wb*t)
     ...: display(f)
     ...: t1 = solve(wb*t-pi, t)
     ...: display(t1)
     ...: f1 = f.subs({t:t1})
     ...: display(f1)
  sin(6*omega_0*t)
  [pi/(6*omega_0)]
  sin(6*omega_0*t)
  </code></pre>
  <p>I expected that the value of the substitution, because there are no more free variables, would be <code>sin(pi)</code> or, even better, <code>0</code>.<br />
  Obviously I'm missing something...</p>
  <hr />
  <p>As exposed by <a href="https://stackoverflow.com/users/9450991/oscar-benjamin">Oscar Benjamin</a> in the <a href="https://stackoverflow.com/a/73593856/2749397">answer below</a> <code>t1</code> is a list — as a matter of fact, I need a coffee.</p>