Why is the post edit form not working in flask?

Question

Flask

Good afternoon! I use the form.populate_obj(post) method. but after editing the post, I will be redirected again to the form for editing this post, but which remained unchanged
Here is the code of the function that should edit the post.

@posts.route('/<slug>/edit/', methods = ['POST', 'GET'])
def edit_post(slug):
    post = Post.query.filter(Post.slug ==slug).first()

    if request.method ==' POST' :
        form = PostForm(formdata=request.form, obj =post)
        form.populate_obj(post)
        db.session.add(post)
        db.session.commit()
        return redirect(url_for('posts.post_detail', slug = post.slug, ))

    form = PostForm(obj=post)
    return render_template('posts/edit.html', post=post, form = form)

Please tell me why my post editing is not working.