Added forward euler in python3 (#203)

Author: sklan

chapters/differential_equations/euler/code/python/euler.py

@@ -0,0 +1,31 @@
+import math
+
+
+def forward_euler(time_step, n):
+    result = [0] * n
+    result[0] = 1
+    for i in range(1, n):
+        result[i] = result[i - 1] - 3 * result[i - 1] * time_step
+    return result
+
+
+def check(result, threshold, time_step):
+    approx = True
+    for i in range(len(result)):
+        solution = math.exp(-3 * i * time_step)
+        if abs(result[i] - solution) > threshold:
+            print(result[i], solution)
+            approx = False
+    return approx
+
+
+def main():
+    time_step = 0.01
+    n = 100
+    threshold = 0.01
+
+    result = forward_euler(time_step, n)
+    approx = check(result, threshold, time_step)
+    print("All values within threshold") if approx else print("Value(s) not in threshold")
+
+main()

chapters/differential_equations/euler/euler.md

@@ -110,6 +110,10 @@ So, this time, let's remove ourselves from any physics and instead solve the fol
 
 Full code for the visualization follows:
 [import, lang:"elm"](code/elm/euler.elm)
+
+{% sample lang="py" %}
+### Python
+[import, lang:"python"](code/python/euler.py)
 {% endmethod %}
 
 <script>